[syzbot] WARNING: suspicious RCU usage in get_timespec64

[syzbot] WARNING: suspicious RCU usage in get_timespec64

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    5e46d1b7 reiserfs: update reiserfs_xattrs_initialized() co..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=1125f831d00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=78ef1d159159890
dashboard link: https://syzkaller.appspot.com/bug?extid=88e4f02896967fe1ab0d

Unfortunately, I don't have any reproducer for this issue yet.

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+88e4f02896967fe1ab0d@syzkaller.appspotmail.com

=============================
WARNING: suspicious RCU usage
5.12.0-rc5-syzkaller #0 Not tainted
-----------------------------
kernel/sched/core.c:8294 Illegal context switch in RCU-sched read-side critical section!

other info that might help us debug this:


rcu_scheduler_active = 2, debug_locks = 0
3 locks held by syz-executor.4/8418:
 #0: 
ffff8880751d2b28
 (
&p->pi_lock
){-.-.}-{2:2}
, at: try_to_wake_up+0x98/0x14a0 kernel/sched/core.c:3345
 #1: 
ffff8880b9d35258
 (
&rq->lock
){-.-.}-{2:2}
, at: rq_lock kernel/sched/sched.h:1321 [inline]
, at: ttwu_queue kernel/sched/core.c:3184 [inline]
, at: try_to_wake_up+0x5e6/0x14a0 kernel/sched/core.c:3464
 #2: ffff8880b9d1f948 (&per_cpu_ptr(group->pcpu, cpu)->seq){-.-.}-{0:0}, at: psi_task_change+0x142/0x220 kernel/sched/psi.c:807

stack backtrace:
CPU: 0 PID: 8418 Comm: syz-executor.4 Not tainted 5.12.0-rc5-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:79 [inline]
 dump_stack+0x141/0x1d7 lib/dump_stack.c:120
 ___might_sleep+0x266/0x2c0 kernel/sched/core.c:8294
 __might_fault+0x6e/0x180 mm/memory.c:5018
 _copy_from_user+0x27/0x180 lib/usercopy.c:13
 copy_from_user include/linux/uaccess.h:192 [inline]
 get_timespec64+0x75/0x220 kernel/time/time.c:787
 __do_sys_clock_nanosleep kernel/time/posix-timers.c:1257 [inline]
 __se_sys_clock_nanosleep kernel/time/posix-timers.c:1245 [inline]
 __x64_sys_clock_nanosleep+0x1bb/0x430 kernel/time/posix-timers.c:1245
 do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xae
RIP: 0033:0x48a621
Code: 24 0c 89 3c 24 48 89 4c 24 18 e8 aa e7 ff ff 4c 8b 54 24 18 48 8b 54 24 10 41 89 c0 8b 74 24 0c 8b 3c 24 b8 e6 00 00 00 0f 05 <44> 89 c7 48 89 04 24 e8 e3 e7 ff ff 48 8b 04 24 eb 97 66 2e 0f 1f
RSP: 002b:00007fffe59fbd50 EFLAGS: 00000293 ORIG_RAX: 00000000000000e6
RAX: ffffffffffffffda RBX: 0000000000000294 RCX: 000000000048a621
RDX: 00007fffe59fbd90 RSI: 0000000000000000 RDI: 0000000000000000
RBP: 00007fffe59fbe2c R08: 0000000000000000 R09: 00007fffe5b8a090
R10: 0000000000000000 R11: 0000000000000293 R12: 0000000000000032
R13: 000000000005717a R14: 0000000000000003 R15: 00007fffe59fbe90


---
This report is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzkaller@googlegroups.com.

syzbot will keep track of this issue. See:
https://goo.gl/tpsmEJ#status for how to communicate with syzbot.

Re: [syzbot] WARNING: suspicious RCU usage in get_timespec64

[Thomas Gleixner]

On Sun, Apr 04 2021 at 12:05, syzbot wrote:

Cc + ...

> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit:    5e46d1b7 reiserfs: update reiserfs_xattrs_initialized() co..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=1125f831d00000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=78ef1d159159890
> dashboard link: https://syzkaller.appspot.com/bug?extid=88e4f02896967fe1ab0d
>
> Unfortunately, I don't have any reproducer for this issue yet.
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+88e4f02896967fe1ab0d@syzkaller.appspotmail.com
>
> =============================
> WARNING: suspicious RCU usage
> 5.12.0-rc5-syzkaller #0 Not tainted
> -----------------------------
> kernel/sched/core.c:8294 Illegal context switch in RCU-sched read-side critical section!
>
> other info that might help us debug this:
>
>
> rcu_scheduler_active = 2, debug_locks = 0
> 3 locks held by syz-executor.4/8418:
>  #0: 
> ffff8880751d2b28
>  (
> &p->pi_lock
> ){-.-.}-{2:2}
> , at: try_to_wake_up+0x98/0x14a0 kernel/sched/core.c:3345
>  #1: 
> ffff8880b9d35258
>  (
> &rq->lock
> ){-.-.}-{2:2}
> , at: rq_lock kernel/sched/sched.h:1321 [inline]
> , at: ttwu_queue kernel/sched/core.c:3184 [inline]
> , at: try_to_wake_up+0x5e6/0x14a0 kernel/sched/core.c:3464
>  #2: ffff8880b9d1f948 (&per_cpu_ptr(group->pcpu, cpu)->seq){-.-.}-{0:0}, at: psi_task_change+0x142/0x220 kernel/sched/psi.c:807
>
> stack backtrace:
> CPU: 0 PID: 8418 Comm: syz-executor.4 Not tainted 5.12.0-rc5-syzkaller #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> Call Trace:
>  __dump_stack lib/dump_stack.c:79 [inline]
>  dump_stack+0x141/0x1d7 lib/dump_stack.c:120
>  ___might_sleep+0x266/0x2c0 kernel/sched/core.c:8294
>  __might_fault+0x6e/0x180 mm/memory.c:5018
>  _copy_from_user+0x27/0x180 lib/usercopy.c:13
>  copy_from_user include/linux/uaccess.h:192 [inline]
>  get_timespec64+0x75/0x220 kernel/time/time.c:787
>  __do_sys_clock_nanosleep kernel/time/posix-timers.c:1257 [inline]
>  __se_sys_clock_nanosleep kernel/time/posix-timers.c:1245 [inline]
>  __x64_sys_clock_nanosleep+0x1bb/0x430 kernel/time/posix-timers.c:1245
>  do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
>  entry_SYSCALL_64_after_hwframe+0x44/0xae
> RIP: 0033:0x48a621
> Code: 24 0c 89 3c 24 48 89 4c 24 18 e8 aa e7 ff ff 4c 8b 54 24 18 48 8b 54 24 10 41 89 c0 8b 74 24 0c 8b 3c 24 b8 e6 00 00 00 0f 05 <44> 89 c7 48 89 04 24 e8 e3 e7 ff ff 48 8b 04 24 eb 97 66 2e 0f 1f
> RSP: 002b:00007fffe59fbd50 EFLAGS: 00000293 ORIG_RAX: 00000000000000e6
> RAX: ffffffffffffffda RBX: 0000000000000294 RCX: 000000000048a621
> RDX: 00007fffe59fbd90 RSI: 0000000000000000 RDI: 0000000000000000
> RBP: 00007fffe59fbe2c R08: 0000000000000000 R09: 00007fffe5b8a090
> R10: 0000000000000000 R11: 0000000000000293 R12: 0000000000000032
> R13: 000000000005717a R14: 0000000000000003 R15: 00007fffe59fbe90
>
>
> ---
> This report is generated by a bot. It may contain errors.
> See https://goo.gl/tpsmEJ for more information about syzbot.
> syzbot engineers can be reached at syzkaller@googlegroups.com.
>
> syzbot will keep track of this issue. See:
> https://goo.gl/tpsmEJ#status for how to communicate with syzbot.

Re: [syzbot] WARNING: suspicious RCU usage in get_timespec64

[Paul E. McKenney]

On Sun, Apr 04, 2021 at 10:38:41PM +0200, Thomas Gleixner wrote:
> On Sun, Apr 04 2021 at 12:05, syzbot wrote:
> 
> Cc + ...

And a couple more...

> > Hello,
> >
> > syzbot found the following issue on:
> >
> > HEAD commit:    5e46d1b7 reiserfs: update reiserfs_xattrs_initialized() co..
> > git tree:       upstream
> > console output: https://syzkaller.appspot.com/x/log.txt?x=1125f831d00000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=78ef1d159159890
> > dashboard link: https://syzkaller.appspot.com/bug?extid=88e4f02896967fe1ab0d
> >
> > Unfortunately, I don't have any reproducer for this issue yet.
> >
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+88e4f02896967fe1ab0d@syzkaller.appspotmail.com
> >
> > =============================
> > WARNING: suspicious RCU usage
> > 5.12.0-rc5-syzkaller #0 Not tainted
> > -----------------------------
> > kernel/sched/core.c:8294 Illegal context switch in RCU-sched read-side critical section!
> >
> > other info that might help us debug this:
> >
> >
> > rcu_scheduler_active = 2, debug_locks = 0
> > 3 locks held by syz-executor.4/8418:
> >  #0: 
> > ffff8880751d2b28
> >  (
> > &p->pi_lock
> > ){-.-.}-{2:2}
> > , at: try_to_wake_up+0x98/0x14a0 kernel/sched/core.c:3345
> >  #1: 
> > ffff8880b9d35258
> >  (
> > &rq->lock
> > ){-.-.}-{2:2}
> > , at: rq_lock kernel/sched/sched.h:1321 [inline]
> > , at: ttwu_queue kernel/sched/core.c:3184 [inline]
> > , at: try_to_wake_up+0x5e6/0x14a0 kernel/sched/core.c:3464
> >  #2: ffff8880b9d1f948 (&per_cpu_ptr(group->pcpu, cpu)->seq){-.-.}-{0:0}, at: psi_task_change+0x142/0x220 kernel/sched/psi.c:807

This looks similar to syzbot+dde0cc33951735441301@syzkaller.appspotmail.com
in that rcu_sleep_check() sees an RCU lock held, but the later call to
lockdep_print_held_locks() does not.  Did something change recently that
could let the ->lockdep_depth counter get out of sync with the actual
number of locks held?

							Thanx, Paul

> > stack backtrace:
> > CPU: 0 PID: 8418 Comm: syz-executor.4 Not tainted 5.12.0-rc5-syzkaller #0
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> > Call Trace:
> >  __dump_stack lib/dump_stack.c:79 [inline]
> >  dump_stack+0x141/0x1d7 lib/dump_stack.c:120
> >  ___might_sleep+0x266/0x2c0 kernel/sched/core.c:8294
> >  __might_fault+0x6e/0x180 mm/memory.c:5018
> >  _copy_from_user+0x27/0x180 lib/usercopy.c:13
> >  copy_from_user include/linux/uaccess.h:192 [inline]
> >  get_timespec64+0x75/0x220 kernel/time/time.c:787
> >  __do_sys_clock_nanosleep kernel/time/posix-timers.c:1257 [inline]
> >  __se_sys_clock_nanosleep kernel/time/posix-timers.c:1245 [inline]
> >  __x64_sys_clock_nanosleep+0x1bb/0x430 kernel/time/posix-timers.c:1245
> >  do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
> >  entry_SYSCALL_64_after_hwframe+0x44/0xae
> > RIP: 0033:0x48a621
> > Code: 24 0c 89 3c 24 48 89 4c 24 18 e8 aa e7 ff ff 4c 8b 54 24 18 48 8b 54 24 10 41 89 c0 8b 74 24 0c 8b 3c 24 b8 e6 00 00 00 0f 05 <44> 89 c7 48 89 04 24 e8 e3 e7 ff ff 48 8b 04 24 eb 97 66 2e 0f 1f
> > RSP: 002b:00007fffe59fbd50 EFLAGS: 00000293 ORIG_RAX: 00000000000000e6
> > RAX: ffffffffffffffda RBX: 0000000000000294 RCX: 000000000048a621
> > RDX: 00007fffe59fbd90 RSI: 0000000000000000 RDI: 0000000000000000
> > RBP: 00007fffe59fbe2c R08: 0000000000000000 R09: 00007fffe5b8a090
> > R10: 0000000000000000 R11: 0000000000000293 R12: 0000000000000032
> > R13: 000000000005717a R14: 0000000000000003 R15: 00007fffe59fbe90
> >
> >
> > ---
> > This report is generated by a bot. It may contain errors.
> > See https://goo.gl/tpsmEJ for more information about syzbot.
> > syzbot engineers can be reached at syzkaller@googlegroups.com.
> >
> > syzbot will keep track of this issue. See:
> > https://goo.gl/tpsmEJ#status for how to communicate with syzbot.

Re: [syzbot] WARNING: suspicious RCU usage in get_timespec64

[Matthew Wilcox]

On Sun, Apr 04, 2021 at 02:40:30PM -0700, Paul E. McKenney wrote:
> On Sun, Apr 04, 2021 at 10:38:41PM +0200, Thomas Gleixner wrote:
> > On Sun, Apr 04 2021 at 12:05, syzbot wrote:
> > 
> > Cc + ...
> 
> And a couple more...
> 
> > > Hello,
> > >
> > > syzbot found the following issue on:
> > >
> > > HEAD commit:    5e46d1b7 reiserfs: update reiserfs_xattrs_initialized() co..
> > > git tree:       upstream
> > > console output: https://syzkaller.appspot.com/x/log.txt?x=1125f831d00000
> > > kernel config:  https://syzkaller.appspot.com/x/.config?x=78ef1d159159890
> > > dashboard link: https://syzkaller.appspot.com/bug?extid=88e4f02896967fe1ab0d
> > >
> > > Unfortunately, I don't have any reproducer for this issue yet.
> > >
> > > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > > Reported-by: syzbot+88e4f02896967fe1ab0d@syzkaller.appspotmail.com
> > >
> > > =============================
> > > WARNING: suspicious RCU usage
> > > 5.12.0-rc5-syzkaller #0 Not tainted
> > > -----------------------------
> > > kernel/sched/core.c:8294 Illegal context switch in RCU-sched read-side critical section!
> > >
> > > other info that might help us debug this:
> > >
> > >
> > > rcu_scheduler_active = 2, debug_locks = 0
> > > 3 locks held by syz-executor.4/8418:
> > >  #0: 
> > > ffff8880751d2b28
> > >  (
> > > &p->pi_lock
> > > ){-.-.}-{2:2}
> > > , at: try_to_wake_up+0x98/0x14a0 kernel/sched/core.c:3345
> > >  #1: 
> > > ffff8880b9d35258
> > >  (
> > > &rq->lock
> > > ){-.-.}-{2:2}
> > > , at: rq_lock kernel/sched/sched.h:1321 [inline]
> > > , at: ttwu_queue kernel/sched/core.c:3184 [inline]
> > > , at: try_to_wake_up+0x5e6/0x14a0 kernel/sched/core.c:3464
> > >  #2: ffff8880b9d1f948 (&per_cpu_ptr(group->pcpu, cpu)->seq){-.-.}-{0:0}, at: psi_task_change+0x142/0x220 kernel/sched/psi.c:807
> 
> This looks similar to syzbot+dde0cc33951735441301@syzkaller.appspotmail.com
> in that rcu_sleep_check() sees an RCU lock held, but the later call to
> lockdep_print_held_locks() does not.  Did something change recently that
> could let the ->lockdep_depth counter get out of sync with the actual
> number of locks held?

Dmitri had a different theory here:

https://groups.google.com/g/syzkaller-bugs/c/FmYvfZCZzqA/m/nc2CXUgsAgAJ

Re: [syzbot] WARNING: suspicious RCU usage in get_timespec64

[Paul E. McKenney]

On Mon, Apr 05, 2021 at 04:08:55AM +0100, Matthew Wilcox wrote:
> On Sun, Apr 04, 2021 at 02:40:30PM -0700, Paul E. McKenney wrote:
> > On Sun, Apr 04, 2021 at 10:38:41PM +0200, Thomas Gleixner wrote:
> > > On Sun, Apr 04 2021 at 12:05, syzbot wrote:
> > > 
> > > Cc + ...
> > 
> > And a couple more...
> > 
> > > > Hello,
> > > >
> > > > syzbot found the following issue on:
> > > >
> > > > HEAD commit:    5e46d1b7 reiserfs: update reiserfs_xattrs_initialized() co..
> > > > git tree:       upstream
> > > > console output: https://syzkaller.appspot.com/x/log.txt?x=1125f831d00000
> > > > kernel config:  https://syzkaller.appspot.com/x/.config?x=78ef1d159159890
> > > > dashboard link: https://syzkaller.appspot.com/bug?extid=88e4f02896967fe1ab0d
> > > >
> > > > Unfortunately, I don't have any reproducer for this issue yet.
> > > >
> > > > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > > > Reported-by: syzbot+88e4f02896967fe1ab0d@syzkaller.appspotmail.com
> > > >
> > > > =============================
> > > > WARNING: suspicious RCU usage
> > > > 5.12.0-rc5-syzkaller #0 Not tainted
> > > > -----------------------------
> > > > kernel/sched/core.c:8294 Illegal context switch in RCU-sched read-side critical section!
> > > >
> > > > other info that might help us debug this:
> > > >
> > > >
> > > > rcu_scheduler_active = 2, debug_locks = 0
> > > > 3 locks held by syz-executor.4/8418:
> > > >  #0: 
> > > > ffff8880751d2b28
> > > >  (
> > > > &p->pi_lock
> > > > ){-.-.}-{2:2}
> > > > , at: try_to_wake_up+0x98/0x14a0 kernel/sched/core.c:3345
> > > >  #1: 
> > > > ffff8880b9d35258
> > > >  (
> > > > &rq->lock
> > > > ){-.-.}-{2:2}
> > > > , at: rq_lock kernel/sched/sched.h:1321 [inline]
> > > > , at: ttwu_queue kernel/sched/core.c:3184 [inline]
> > > > , at: try_to_wake_up+0x5e6/0x14a0 kernel/sched/core.c:3464
> > > >  #2: ffff8880b9d1f948 (&per_cpu_ptr(group->pcpu, cpu)->seq){-.-.}-{0:0}, at: psi_task_change+0x142/0x220 kernel/sched/psi.c:807
> > 
> > This looks similar to syzbot+dde0cc33951735441301@syzkaller.appspotmail.com
> > in that rcu_sleep_check() sees an RCU lock held, but the later call to
> > lockdep_print_held_locks() does not.  Did something change recently that
> > could let the ->lockdep_depth counter get out of sync with the actual
> > number of locks held?
> 
> Dmitri had a different theory here:
> 
> https://groups.google.com/g/syzkaller-bugs/c/FmYvfZCZzqA/m/nc2CXUgsAgAJ

There is always room for more than one bug.  ;-)

He says "one-off false positives".  I was afraid of that...

							Thanx, Paul

Re: [syzbot] WARNING: suspicious RCU usage in get_timespec64

[Paul E. McKenney]

On Sun, Apr 04, 2021 at 09:01:25PM -0700, Paul E. McKenney wrote:
> On Mon, Apr 05, 2021 at 04:08:55AM +0100, Matthew Wilcox wrote:
> > On Sun, Apr 04, 2021 at 02:40:30PM -0700, Paul E. McKenney wrote:
> > > On Sun, Apr 04, 2021 at 10:38:41PM +0200, Thomas Gleixner wrote:
> > > > On Sun, Apr 04 2021 at 12:05, syzbot wrote:
> > > > 
> > > > Cc + ...
> > > 
> > > And a couple more...
> > > 
> > > > > Hello,
> > > > >
> > > > > syzbot found the following issue on:
> > > > >
> > > > > HEAD commit:    5e46d1b7 reiserfs: update reiserfs_xattrs_initialized() co..
> > > > > git tree:       upstream
> > > > > console output: https://syzkaller.appspot.com/x/log.txt?x=1125f831d00000
> > > > > kernel config:  https://syzkaller.appspot.com/x/.config?x=78ef1d159159890
> > > > > dashboard link: https://syzkaller.appspot.com/bug?extid=88e4f02896967fe1ab0d
> > > > >
> > > > > Unfortunately, I don't have any reproducer for this issue yet.
> > > > >
> > > > > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > > > > Reported-by: syzbot+88e4f02896967fe1ab0d@syzkaller.appspotmail.com
> > > > >
> > > > > =============================
> > > > > WARNING: suspicious RCU usage
> > > > > 5.12.0-rc5-syzkaller #0 Not tainted
> > > > > -----------------------------
> > > > > kernel/sched/core.c:8294 Illegal context switch in RCU-sched read-side critical section!
> > > > >
> > > > > other info that might help us debug this:
> > > > >
> > > > >
> > > > > rcu_scheduler_active = 2, debug_locks = 0
> > > > > 3 locks held by syz-executor.4/8418:
> > > > >  #0: 
> > > > > ffff8880751d2b28
> > > > >  (
> > > > > &p->pi_lock
> > > > > ){-.-.}-{2:2}
> > > > > , at: try_to_wake_up+0x98/0x14a0 kernel/sched/core.c:3345
> > > > >  #1: 
> > > > > ffff8880b9d35258
> > > > >  (
> > > > > &rq->lock
> > > > > ){-.-.}-{2:2}
> > > > > , at: rq_lock kernel/sched/sched.h:1321 [inline]
> > > > > , at: ttwu_queue kernel/sched/core.c:3184 [inline]
> > > > > , at: try_to_wake_up+0x5e6/0x14a0 kernel/sched/core.c:3464
> > > > >  #2: ffff8880b9d1f948 (&per_cpu_ptr(group->pcpu, cpu)->seq){-.-.}-{0:0}, at: psi_task_change+0x142/0x220 kernel/sched/psi.c:807
> > > 
> > > This looks similar to syzbot+dde0cc33951735441301@syzkaller.appspotmail.com
> > > in that rcu_sleep_check() sees an RCU lock held, but the later call to
> > > lockdep_print_held_locks() does not.  Did something change recently that
> > > could let the ->lockdep_depth counter get out of sync with the actual
> > > number of locks held?
> > 
> > Dmitri had a different theory here:
> > 
> > https://groups.google.com/g/syzkaller-bugs/c/FmYvfZCZzqA/m/nc2CXUgsAgAJ
> 
> There is always room for more than one bug.  ;-)
> 
> He says "one-off false positives".  I was afraid of that...

And both the examples I have been copied on today are consistent with
debug_locks getting zeroed (e.g., via a call to __debug_locks_off())
in the midst of a call to rcu_sleep_check().  But I would expect to see
a panic or another splat if that were to happen.

Dmitry's example did have an additional splat, but I would expect the
RCU-related one to come second.  Again, there is always room for more
than one bug.

On the other hand, there are a lot more callers to debug_locks_off()
than there were last I looked into this.  And both of these splats
are consistent with an interrupt in the middle of rcu_sleep_check(),
and that interrupt's handler invoking debug_locks_off(), but without
printing anything to the console.  Does that sequence of events ring a
bell for anyone?

If this is the new normal, I could make RCU_LOCKDEP_WARN() recheck
debug_lockdep_rcu_enabled() after evaluating the condition, but with
a memory barrier immediately before the recheck.  But I am not at all
excited by doing this on speculation.  Especially given that doing
so might be covering up some other bug.

							Thanx, Paul

Re: [syzbot] WARNING: suspicious RCU usage in get_timespec64

[Boqun Feng]

On Sun, Apr 04, 2021 at 09:30:38PM -0700, Paul E. McKenney wrote:
> On Sun, Apr 04, 2021 at 09:01:25PM -0700, Paul E. McKenney wrote:
> > On Mon, Apr 05, 2021 at 04:08:55AM +0100, Matthew Wilcox wrote:
> > > On Sun, Apr 04, 2021 at 02:40:30PM -0700, Paul E. McKenney wrote:
> > > > On Sun, Apr 04, 2021 at 10:38:41PM +0200, Thomas Gleixner wrote:
> > > > > On Sun, Apr 04 2021 at 12:05, syzbot wrote:
> > > > > 
> > > > > Cc + ...
> > > > 
> > > > And a couple more...
> > > > 
> > > > > > Hello,
> > > > > >
> > > > > > syzbot found the following issue on:
> > > > > >
> > > > > > HEAD commit:    5e46d1b7 reiserfs: update reiserfs_xattrs_initialized() co..
> > > > > > git tree:       upstream
> > > > > > console output: https://syzkaller.appspot.com/x/log.txt?x=1125f831d00000
> > > > > > kernel config:  https://syzkaller.appspot.com/x/.config?x=78ef1d159159890
> > > > > > dashboard link: https://syzkaller.appspot.com/bug?extid=88e4f02896967fe1ab0d
> > > > > >
> > > > > > Unfortunately, I don't have any reproducer for this issue yet.
> > > > > >
> > > > > > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > > > > > Reported-by: syzbot+88e4f02896967fe1ab0d@syzkaller.appspotmail.com
> > > > > >
> > > > > > =============================
> > > > > > WARNING: suspicious RCU usage
> > > > > > 5.12.0-rc5-syzkaller #0 Not tainted
> > > > > > -----------------------------
> > > > > > kernel/sched/core.c:8294 Illegal context switch in RCU-sched read-side critical section!
> > > > > >
> > > > > > other info that might help us debug this:
> > > > > >
> > > > > >
> > > > > > rcu_scheduler_active = 2, debug_locks = 0
> > > > > > 3 locks held by syz-executor.4/8418:
> > > > > >  #0: 
> > > > > > ffff8880751d2b28
> > > > > >  (
> > > > > > &p->pi_lock
> > > > > > ){-.-.}-{2:2}
> > > > > > , at: try_to_wake_up+0x98/0x14a0 kernel/sched/core.c:3345
> > > > > >  #1: 
> > > > > > ffff8880b9d35258
> > > > > >  (
> > > > > > &rq->lock
> > > > > > ){-.-.}-{2:2}
> > > > > > , at: rq_lock kernel/sched/sched.h:1321 [inline]
> > > > > > , at: ttwu_queue kernel/sched/core.c:3184 [inline]
> > > > > > , at: try_to_wake_up+0x5e6/0x14a0 kernel/sched/core.c:3464
> > > > > >  #2: ffff8880b9d1f948 (&per_cpu_ptr(group->pcpu, cpu)->seq){-.-.}-{0:0}, at: psi_task_change+0x142/0x220 kernel/sched/psi.c:807
> > > > 
> > > > This looks similar to syzbot+dde0cc33951735441301@syzkaller.appspotmail.com
> > > > in that rcu_sleep_check() sees an RCU lock held, but the later call to
> > > > lockdep_print_held_locks() does not.  Did something change recently that
> > > > could let the ->lockdep_depth counter get out of sync with the actual
> > > > number of locks held?
> > > 
> > > Dmitri had a different theory here:
> > > 
> > > https://groups.google.com/g/syzkaller-bugs/c/FmYvfZCZzqA/m/nc2CXUgsAgAJ
> > 
> > There is always room for more than one bug.  ;-)
> > 
> > He says "one-off false positives".  I was afraid of that...
> 
> And both the examples I have been copied on today are consistent with
> debug_locks getting zeroed (e.g., via a call to __debug_locks_off())
> in the midst of a call to rcu_sleep_check().  But I would expect to see
> a panic or another splat if that were to happen.
> 
> Dmitry's example did have an additional splat, but I would expect the
> RCU-related one to come second.  Again, there is always room for more
> than one bug.
> 
> On the other hand, there are a lot more callers to debug_locks_off()
> than there were last I looked into this.  And both of these splats
> are consistent with an interrupt in the middle of rcu_sleep_check(),
> and that interrupt's handler invoking debug_locks_off(), but without
> printing anything to the console.  Does that sequence of events ring a
> bell for anyone?
> 
> If this is the new normal, I could make RCU_LOCKDEP_WARN() recheck
> debug_lockdep_rcu_enabled() after evaluating the condition, but with
> a memory barrier immediately before the recheck.  But I am not at all
> excited by doing this on speculation.  Especially given that doing
> so might be covering up some other bug.
> 

Just check the original console log and find:

[  356.696686][ T8418] =============================
[  356.696692][ T8418] WARNING: suspicious RCU usage
[  356.700193][T14782] ====================================
[  356.704548][ T8418] 5.12.0-rc5-syzkaller #0 Not tainted
[  356.729981][ T8418] -----------------------------
[  356.732473][T14782] WARNING: iou-sqp-14780/14782 still has locks held!

, so there are two warnnings here, one is from lockdep_rcu_suspisous()
and the other is from print_held_locks_bug(). I think this is what
happened:

in RCU_LOCKDEP_WARN():

	if (debug_lockdep_rcu_enabled() // this is true and at this time debug_locks = 1
	<interrupted>
	// lockdep detects a lock bug, set debug_locks = 0
	<swicth back>
	    && !__warned // true
	    && (c))      // "c" is a lock_is_held(), which will always returns true if debug_locks == 0!

the cause of the problem is that RCU_LOCKDEP_WARN() in fact read
debug_locks twice and get different values.

But if you change the ordering of two reads, probably can avoid the
problem:
	
First read:
	lock_is_held(); // true if 1) lock is really held or 2) lockdep is off

Second read:
	debug_lockdep_rcu_enabled(); // if lockdep is not off, we know
				     // that the first read got correct
				     // value, otherwise we just ignore
				     // the first read, because either
				     // there is a bug reported between
				     // two reads, or lockdep is already
				     // off when the first read happens.

So maybe something below:

Regards,
Boqun

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index bd04f722714f..d11bee5d9347 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -315,7 +315,7 @@ static inline int rcu_read_lock_any_held(void)
 #define RCU_LOCKDEP_WARN(c, s)						\
 	do {								\
 		static bool __section(".data.unlikely") __warned;	\
-		if (debug_lockdep_rcu_enabled() && !__warned && (c)) {	\
+		if ((c) && debug_lockdep_rcu_enabled() && !__warned) {	\
 			__warned = true;				\
 			lockdep_rcu_suspicious(__FILE__, __LINE__, s);	\
 		}							\

> 							Thanx, Paul

Re: [syzbot] WARNING: suspicious RCU usage in get_timespec64

[Paul E. McKenney]

On Mon, Apr 05, 2021 at 01:23:30PM +0800, Boqun Feng wrote:
> On Sun, Apr 04, 2021 at 09:30:38PM -0700, Paul E. McKenney wrote:
> > On Sun, Apr 04, 2021 at 09:01:25PM -0700, Paul E. McKenney wrote:
> > > On Mon, Apr 05, 2021 at 04:08:55AM +0100, Matthew Wilcox wrote:
> > > > On Sun, Apr 04, 2021 at 02:40:30PM -0700, Paul E. McKenney wrote:
> > > > > On Sun, Apr 04, 2021 at 10:38:41PM +0200, Thomas Gleixner wrote:
> > > > > > On Sun, Apr 04 2021 at 12:05, syzbot wrote:
> > > > > > 
> > > > > > Cc + ...
> > > > > 
> > > > > And a couple more...
> > > > > 
> > > > > > > Hello,
> > > > > > >
> > > > > > > syzbot found the following issue on:
> > > > > > >
> > > > > > > HEAD commit:    5e46d1b7 reiserfs: update reiserfs_xattrs_initialized() co..
> > > > > > > git tree:       upstream
> > > > > > > console output: https://syzkaller.appspot.com/x/log.txt?x=1125f831d00000
> > > > > > > kernel config:  https://syzkaller.appspot.com/x/.config?x=78ef1d159159890
> > > > > > > dashboard link: https://syzkaller.appspot.com/bug?extid=88e4f02896967fe1ab0d
> > > > > > >
> > > > > > > Unfortunately, I don't have any reproducer for this issue yet.
> > > > > > >
> > > > > > > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > > > > > > Reported-by: syzbot+88e4f02896967fe1ab0d@syzkaller.appspotmail.com
> > > > > > >
> > > > > > > =============================
> > > > > > > WARNING: suspicious RCU usage
> > > > > > > 5.12.0-rc5-syzkaller #0 Not tainted
> > > > > > > -----------------------------
> > > > > > > kernel/sched/core.c:8294 Illegal context switch in RCU-sched read-side critical section!
> > > > > > >
> > > > > > > other info that might help us debug this:
> > > > > > >
> > > > > > >
> > > > > > > rcu_scheduler_active = 2, debug_locks = 0
> > > > > > > 3 locks held by syz-executor.4/8418:
> > > > > > >  #0: 
> > > > > > > ffff8880751d2b28
> > > > > > >  (
> > > > > > > &p->pi_lock
> > > > > > > ){-.-.}-{2:2}
> > > > > > > , at: try_to_wake_up+0x98/0x14a0 kernel/sched/core.c:3345
> > > > > > >  #1: 
> > > > > > > ffff8880b9d35258
> > > > > > >  (
> > > > > > > &rq->lock
> > > > > > > ){-.-.}-{2:2}
> > > > > > > , at: rq_lock kernel/sched/sched.h:1321 [inline]
> > > > > > > , at: ttwu_queue kernel/sched/core.c:3184 [inline]
> > > > > > > , at: try_to_wake_up+0x5e6/0x14a0 kernel/sched/core.c:3464
> > > > > > >  #2: ffff8880b9d1f948 (&per_cpu_ptr(group->pcpu, cpu)->seq){-.-.}-{0:0}, at: psi_task_change+0x142/0x220 kernel/sched/psi.c:807
> > > > > 
> > > > > This looks similar to syzbot+dde0cc33951735441301@syzkaller.appspotmail.com
> > > > > in that rcu_sleep_check() sees an RCU lock held, but the later call to
> > > > > lockdep_print_held_locks() does not.  Did something change recently that
> > > > > could let the ->lockdep_depth counter get out of sync with the actual
> > > > > number of locks held?
> > > > 
> > > > Dmitri had a different theory here:
> > > > 
> > > > https://groups.google.com/g/syzkaller-bugs/c/FmYvfZCZzqA/m/nc2CXUgsAgAJ
> > > 
> > > There is always room for more than one bug.  ;-)
> > > 
> > > He says "one-off false positives".  I was afraid of that...
> > 
> > And both the examples I have been copied on today are consistent with
> > debug_locks getting zeroed (e.g., via a call to __debug_locks_off())
> > in the midst of a call to rcu_sleep_check().  But I would expect to see
> > a panic or another splat if that were to happen.
> > 
> > Dmitry's example did have an additional splat, but I would expect the
> > RCU-related one to come second.  Again, there is always room for more
> > than one bug.
> > 
> > On the other hand, there are a lot more callers to debug_locks_off()
> > than there were last I looked into this.  And both of these splats
> > are consistent with an interrupt in the middle of rcu_sleep_check(),
> > and that interrupt's handler invoking debug_locks_off(), but without
> > printing anything to the console.  Does that sequence of events ring a
> > bell for anyone?
> > 
> > If this is the new normal, I could make RCU_LOCKDEP_WARN() recheck
> > debug_lockdep_rcu_enabled() after evaluating the condition, but with
> > a memory barrier immediately before the recheck.  But I am not at all
> > excited by doing this on speculation.  Especially given that doing
> > so might be covering up some other bug.
> > 
> 
> Just check the original console log and find:
> 
> [  356.696686][ T8418] =============================
> [  356.696692][ T8418] WARNING: suspicious RCU usage
> [  356.700193][T14782] ====================================
> [  356.704548][ T8418] 5.12.0-rc5-syzkaller #0 Not tainted
> [  356.729981][ T8418] -----------------------------
> [  356.732473][T14782] WARNING: iou-sqp-14780/14782 still has locks held!
> 
> , so there are two warnnings here, one is from lockdep_rcu_suspisous()
> and the other is from print_held_locks_bug(). I think this is what
> happened:
> 
> in RCU_LOCKDEP_WARN():
> 
> 	if (debug_lockdep_rcu_enabled() // this is true and at this time debug_locks = 1
> 	<interrupted>
> 	// lockdep detects a lock bug, set debug_locks = 0
> 	<swicth back>
> 	    && !__warned // true
> 	    && (c))      // "c" is a lock_is_held(), which will always returns true if debug_locks == 0!
> 
> the cause of the problem is that RCU_LOCKDEP_WARN() in fact read
> debug_locks twice and get different values.
> 
> But if you change the ordering of two reads, probably can avoid the
> problem:
> 	
> First read:
> 	lock_is_held(); // true if 1) lock is really held or 2) lockdep is off
> 
> Second read:
> 	debug_lockdep_rcu_enabled(); // if lockdep is not off, we know
> 				     // that the first read got correct
> 				     // value, otherwise we just ignore
> 				     // the first read, because either
> 				     // there is a bug reported between
> 				     // two reads, or lockdep is already
> 				     // off when the first read happens.
> 
> So maybe something below:
> 
> Regards,
> Boqun
> 
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index bd04f722714f..d11bee5d9347 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -315,7 +315,7 @@ static inline int rcu_read_lock_any_held(void)
>  #define RCU_LOCKDEP_WARN(c, s)						\
>  	do {								\
>  		static bool __section(".data.unlikely") __warned;	\
> -		if (debug_lockdep_rcu_enabled() && !__warned && (c)) {	\
> +		if ((c) && debug_lockdep_rcu_enabled() && !__warned) {	\
>  			__warned = true;				\
>  			lockdep_rcu_suspicious(__FILE__, __LINE__, s);	\
>  		}							\

Good point -- if we check debug_lockdep_rcu_enabled() after the condition,
then we will reject false positives in cases where debug_locks was switched
to zero out from under us.

However, we do need ordering.  The "c" usually contains lock_is_held(),
which also checks debug_locks, but from some other translation unit.
Back in the day, the translation-unit boundaries would provide the needed
ordering, but LTO...

In addition, the "debug_locks = 0" was originally supposed to be a hint
that the report might be a false positive.  It is clear that this needs
to be made explicit.

Taking all this together, how about the following?  (The intent is
that the changes to lockdep_rcu_suspicious() will be in a separate
commit.)

							Thanx, Paul

------------------------------------------------------------------------

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 9455476..1199ffd 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -315,7 +315,7 @@ static inline int rcu_read_lock_any_held(void)
 #define RCU_LOCKDEP_WARN(c, s)						\
 	do {								\
 		static bool __section(".data.unlikely") __warned;	\
-		if (debug_lockdep_rcu_enabled() && !__warned && (c)) {	\
+		if ((c) && debug_lockdep_rcu_enabled() && !__warned) {	\
 			__warned = true;				\
 			lockdep_rcu_suspicious(__FILE__, __LINE__, s);	\
 		}							\
diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index c6d0c1dc..80065cd 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -6387,6 +6387,7 @@ asmlinkage __visible void lockdep_sys_exit(void)
 void lockdep_rcu_suspicious(const char *file, const int line, const char *s)
 {
 	struct task_struct *curr = current;
+	int dl = READ_ONCE(debug_locks);
 
 	/* Note: the following can be executed concurrently, so be careful. */
 	pr_warn("\n");
@@ -6396,11 +6397,12 @@ void lockdep_rcu_suspicious(const char *file, const int line, const char *s)
 	pr_warn("-----------------------------\n");
 	pr_warn("%s:%d %s!\n", file, line, s);
 	pr_warn("\nother info that might help us debug this:\n\n");
-	pr_warn("\n%srcu_scheduler_active = %d, debug_locks = %d\n",
+	pr_warn("\n%srcu_scheduler_active = %d, debug_locks = %d\n%s",
 	       !rcu_lockdep_current_cpu_online()
 			? "RCU used illegally from offline CPU!\n"
 			: "",
-	       rcu_scheduler_active, debug_locks);
+	       rcu_scheduler_active, dl,
+	       dl ? "" : "Possible false positive due to lockdep disabling via debug_locks = 0\n");
 
 	/*
 	 * If a CPU is in the RCU-free window in idle (ie: in the section
diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index b95ae86..dd94a60 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -277,7 +277,7 @@ EXPORT_SYMBOL_GPL(rcu_callback_map);
 
 noinstr int notrace debug_lockdep_rcu_enabled(void)
 {
-	return rcu_scheduler_active != RCU_SCHEDULER_INACTIVE && debug_locks &&
+	return rcu_scheduler_active != RCU_SCHEDULER_INACTIVE && READ_ONCE(debug_locks) &&
 	       current->lockdep_recursion == 0;
 }
 EXPORT_SYMBOL_GPL(debug_lockdep_rcu_enabled);

Re: [syzbot] WARNING: suspicious RCU usage in get_timespec64

[Boqun Feng]

On Mon, Apr 05, 2021 at 10:27:52AM -0700, Paul E. McKenney wrote:
> On Mon, Apr 05, 2021 at 01:23:30PM +0800, Boqun Feng wrote:
> > On Sun, Apr 04, 2021 at 09:30:38PM -0700, Paul E. McKenney wrote:
> > > On Sun, Apr 04, 2021 at 09:01:25PM -0700, Paul E. McKenney wrote:
> > > > On Mon, Apr 05, 2021 at 04:08:55AM +0100, Matthew Wilcox wrote:
> > > > > On Sun, Apr 04, 2021 at 02:40:30PM -0700, Paul E. McKenney wrote:
> > > > > > On Sun, Apr 04, 2021 at 10:38:41PM +0200, Thomas Gleixner wrote:
> > > > > > > On Sun, Apr 04 2021 at 12:05, syzbot wrote:
> > > > > > > 
> > > > > > > Cc + ...
> > > > > > 
> > > > > > And a couple more...
> > > > > > 
> > > > > > > > Hello,
> > > > > > > >
> > > > > > > > syzbot found the following issue on:
> > > > > > > >
> > > > > > > > HEAD commit:    5e46d1b7 reiserfs: update reiserfs_xattrs_initialized() co..
> > > > > > > > git tree:       upstream
> > > > > > > > console output: https://syzkaller.appspot.com/x/log.txt?x=1125f831d00000
> > > > > > > > kernel config:  https://syzkaller.appspot.com/x/.config?x=78ef1d159159890
> > > > > > > > dashboard link: https://syzkaller.appspot.com/bug?extid=88e4f02896967fe1ab0d
> > > > > > > >
> > > > > > > > Unfortunately, I don't have any reproducer for this issue yet.
> > > > > > > >
> > > > > > > > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > > > > > > > Reported-by: syzbot+88e4f02896967fe1ab0d@syzkaller.appspotmail.com
> > > > > > > >
> > > > > > > > =============================
> > > > > > > > WARNING: suspicious RCU usage
> > > > > > > > 5.12.0-rc5-syzkaller #0 Not tainted
> > > > > > > > -----------------------------
> > > > > > > > kernel/sched/core.c:8294 Illegal context switch in RCU-sched read-side critical section!
> > > > > > > >
> > > > > > > > other info that might help us debug this:
> > > > > > > >
> > > > > > > >
> > > > > > > > rcu_scheduler_active = 2, debug_locks = 0
> > > > > > > > 3 locks held by syz-executor.4/8418:
> > > > > > > >  #0: 
> > > > > > > > ffff8880751d2b28
> > > > > > > >  (
> > > > > > > > &p->pi_lock
> > > > > > > > ){-.-.}-{2:2}
> > > > > > > > , at: try_to_wake_up+0x98/0x14a0 kernel/sched/core.c:3345
> > > > > > > >  #1: 
> > > > > > > > ffff8880b9d35258
> > > > > > > >  (
> > > > > > > > &rq->lock
> > > > > > > > ){-.-.}-{2:2}
> > > > > > > > , at: rq_lock kernel/sched/sched.h:1321 [inline]
> > > > > > > > , at: ttwu_queue kernel/sched/core.c:3184 [inline]
> > > > > > > > , at: try_to_wake_up+0x5e6/0x14a0 kernel/sched/core.c:3464
> > > > > > > >  #2: ffff8880b9d1f948 (&per_cpu_ptr(group->pcpu, cpu)->seq){-.-.}-{0:0}, at: psi_task_change+0x142/0x220 kernel/sched/psi.c:807
> > > > > > 
> > > > > > This looks similar to syzbot+dde0cc33951735441301@syzkaller.appspotmail.com
> > > > > > in that rcu_sleep_check() sees an RCU lock held, but the later call to
> > > > > > lockdep_print_held_locks() does not.  Did something change recently that
> > > > > > could let the ->lockdep_depth counter get out of sync with the actual
> > > > > > number of locks held?
> > > > > 
> > > > > Dmitri had a different theory here:
> > > > > 
> > > > > https://groups.google.com/g/syzkaller-bugs/c/FmYvfZCZzqA/m/nc2CXUgsAgAJ
> > > > 
> > > > There is always room for more than one bug.  ;-)
> > > > 
> > > > He says "one-off false positives".  I was afraid of that...
> > > 
> > > And both the examples I have been copied on today are consistent with
> > > debug_locks getting zeroed (e.g., via a call to __debug_locks_off())
> > > in the midst of a call to rcu_sleep_check().  But I would expect to see
> > > a panic or another splat if that were to happen.
> > > 
> > > Dmitry's example did have an additional splat, but I would expect the
> > > RCU-related one to come second.  Again, there is always room for more
> > > than one bug.
> > > 
> > > On the other hand, there are a lot more callers to debug_locks_off()
> > > than there were last I looked into this.  And both of these splats
> > > are consistent with an interrupt in the middle of rcu_sleep_check(),
> > > and that interrupt's handler invoking debug_locks_off(), but without
> > > printing anything to the console.  Does that sequence of events ring a
> > > bell for anyone?
> > > 
> > > If this is the new normal, I could make RCU_LOCKDEP_WARN() recheck
> > > debug_lockdep_rcu_enabled() after evaluating the condition, but with
> > > a memory barrier immediately before the recheck.  But I am not at all
> > > excited by doing this on speculation.  Especially given that doing
> > > so might be covering up some other bug.
> > > 
> > 
> > Just check the original console log and find:
> > 
> > [  356.696686][ T8418] =============================
> > [  356.696692][ T8418] WARNING: suspicious RCU usage
> > [  356.700193][T14782] ====================================
> > [  356.704548][ T8418] 5.12.0-rc5-syzkaller #0 Not tainted
> > [  356.729981][ T8418] -----------------------------
> > [  356.732473][T14782] WARNING: iou-sqp-14780/14782 still has locks held!
> > 
> > , so there are two warnnings here, one is from lockdep_rcu_suspisous()
> > and the other is from print_held_locks_bug(). I think this is what
> > happened:
> > 
> > in RCU_LOCKDEP_WARN():
> > 
> > 	if (debug_lockdep_rcu_enabled() // this is true and at this time debug_locks = 1
> > 	<interrupted>
> > 	// lockdep detects a lock bug, set debug_locks = 0
> > 	<swicth back>
> > 	    && !__warned // true
> > 	    && (c))      // "c" is a lock_is_held(), which will always returns true if debug_locks == 0!
> > 
> > the cause of the problem is that RCU_LOCKDEP_WARN() in fact read
> > debug_locks twice and get different values.
> > 
> > But if you change the ordering of two reads, probably can avoid the
> > problem:
> > 	
> > First read:
> > 	lock_is_held(); // true if 1) lock is really held or 2) lockdep is off
> > 
> > Second read:
> > 	debug_lockdep_rcu_enabled(); // if lockdep is not off, we know
> > 				     // that the first read got correct
> > 				     // value, otherwise we just ignore
> > 				     // the first read, because either
> > 				     // there is a bug reported between
> > 				     // two reads, or lockdep is already
> > 				     // off when the first read happens.
> > 
> > So maybe something below:
> > 
> > Regards,
> > Boqun
> > 
> > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> > index bd04f722714f..d11bee5d9347 100644
> > --- a/include/linux/rcupdate.h
> > +++ b/include/linux/rcupdate.h
> > @@ -315,7 +315,7 @@ static inline int rcu_read_lock_any_held(void)
> >  #define RCU_LOCKDEP_WARN(c, s)						\
> >  	do {								\
> >  		static bool __section(".data.unlikely") __warned;	\
> > -		if (debug_lockdep_rcu_enabled() && !__warned && (c)) {	\
> > +		if ((c) && debug_lockdep_rcu_enabled() && !__warned) {	\
> >  			__warned = true;				\
> >  			lockdep_rcu_suspicious(__FILE__, __LINE__, s);	\
> >  		}							\
> 
> Good point -- if we check debug_lockdep_rcu_enabled() after the condition,
> then we will reject false positives in cases where debug_locks was switched
> to zero out from under us.
> 
> However, we do need ordering.  The "c" usually contains lock_is_held(),
> which also checks debug_locks, but from some other translation unit.
> Back in the day, the translation-unit boundaries would provide the needed
> ordering, but LTO...
> 
> In addition, the "debug_locks = 0" was originally supposed to be a hint
> that the report might be a false positive.  It is clear that this needs
> to be made explicit.
> 
> Taking all this together, how about the following?  (The intent is
> that the changes to lockdep_rcu_suspicious() will be in a separate
> commit.)
> 

Looks good to me ;-)

Regards,
Boqun

> 							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index 9455476..1199ffd 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -315,7 +315,7 @@ static inline int rcu_read_lock_any_held(void)
>  #define RCU_LOCKDEP_WARN(c, s)						\
>  	do {								\
>  		static bool __section(".data.unlikely") __warned;	\
> -		if (debug_lockdep_rcu_enabled() && !__warned && (c)) {	\
> +		if ((c) && debug_lockdep_rcu_enabled() && !__warned) {	\
>  			__warned = true;				\
>  			lockdep_rcu_suspicious(__FILE__, __LINE__, s);	\
>  		}							\
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index c6d0c1dc..80065cd 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -6387,6 +6387,7 @@ asmlinkage __visible void lockdep_sys_exit(void)
>  void lockdep_rcu_suspicious(const char *file, const int line, const char *s)
>  {
>  	struct task_struct *curr = current;
> +	int dl = READ_ONCE(debug_locks);
>  
>  	/* Note: the following can be executed concurrently, so be careful. */
>  	pr_warn("\n");
> @@ -6396,11 +6397,12 @@ void lockdep_rcu_suspicious(const char *file, const int line, const char *s)
>  	pr_warn("-----------------------------\n");
>  	pr_warn("%s:%d %s!\n", file, line, s);
>  	pr_warn("\nother info that might help us debug this:\n\n");
> -	pr_warn("\n%srcu_scheduler_active = %d, debug_locks = %d\n",
> +	pr_warn("\n%srcu_scheduler_active = %d, debug_locks = %d\n%s",
>  	       !rcu_lockdep_current_cpu_online()
>  			? "RCU used illegally from offline CPU!\n"
>  			: "",
> -	       rcu_scheduler_active, debug_locks);
> +	       rcu_scheduler_active, dl,
> +	       dl ? "" : "Possible false positive due to lockdep disabling via debug_locks = 0\n");
>  
>  	/*
>  	 * If a CPU is in the RCU-free window in idle (ie: in the section
> diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
> index b95ae86..dd94a60 100644
> --- a/kernel/rcu/update.c
> +++ b/kernel/rcu/update.c
> @@ -277,7 +277,7 @@ EXPORT_SYMBOL_GPL(rcu_callback_map);
>  
>  noinstr int notrace debug_lockdep_rcu_enabled(void)
>  {
> -	return rcu_scheduler_active != RCU_SCHEDULER_INACTIVE && debug_locks &&
> +	return rcu_scheduler_active != RCU_SCHEDULER_INACTIVE && READ_ONCE(debug_locks) &&
>  	       current->lockdep_recursion == 0;
>  }
>  EXPORT_SYMBOL_GPL(debug_lockdep_rcu_enabled);

Re: [syzbot] WARNING: suspicious RCU usage in get_timespec64

[Paul E. McKenney]

On Tue, Apr 06, 2021 at 07:25:44AM +0800, Boqun Feng wrote:
> On Mon, Apr 05, 2021 at 10:27:52AM -0700, Paul E. McKenney wrote:
> > On Mon, Apr 05, 2021 at 01:23:30PM +0800, Boqun Feng wrote:
> > > On Sun, Apr 04, 2021 at 09:30:38PM -0700, Paul E. McKenney wrote:
> > > > On Sun, Apr 04, 2021 at 09:01:25PM -0700, Paul E. McKenney wrote:
> > > > > On Mon, Apr 05, 2021 at 04:08:55AM +0100, Matthew Wilcox wrote:
> > > > > > On Sun, Apr 04, 2021 at 02:40:30PM -0700, Paul E. McKenney wrote:
> > > > > > > On Sun, Apr 04, 2021 at 10:38:41PM +0200, Thomas Gleixner wrote:
> > > > > > > > On Sun, Apr 04 2021 at 12:05, syzbot wrote:
> > > > > > > > 
> > > > > > > > Cc + ...
> > > > > > > 
> > > > > > > And a couple more...
> > > > > > > 
> > > > > > > > > Hello,
> > > > > > > > >
> > > > > > > > > syzbot found the following issue on:
> > > > > > > > >
> > > > > > > > > HEAD commit:    5e46d1b7 reiserfs: update reiserfs_xattrs_initialized() co..
> > > > > > > > > git tree:       upstream
> > > > > > > > > console output: https://syzkaller.appspot.com/x/log.txt?x=1125f831d00000
> > > > > > > > > kernel config:  https://syzkaller.appspot.com/x/.config?x=78ef1d159159890
> > > > > > > > > dashboard link: https://syzkaller.appspot.com/bug?extid=88e4f02896967fe1ab0d
> > > > > > > > >
> > > > > > > > > Unfortunately, I don't have any reproducer for this issue yet.
> > > > > > > > >
> > > > > > > > > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > > > > > > > > Reported-by: syzbot+88e4f02896967fe1ab0d@syzkaller.appspotmail.com
> > > > > > > > >
> > > > > > > > > =============================
> > > > > > > > > WARNING: suspicious RCU usage
> > > > > > > > > 5.12.0-rc5-syzkaller #0 Not tainted
> > > > > > > > > -----------------------------
> > > > > > > > > kernel/sched/core.c:8294 Illegal context switch in RCU-sched read-side critical section!
> > > > > > > > >
> > > > > > > > > other info that might help us debug this:
> > > > > > > > >
> > > > > > > > >
> > > > > > > > > rcu_scheduler_active = 2, debug_locks = 0
> > > > > > > > > 3 locks held by syz-executor.4/8418:
> > > > > > > > >  #0: 
> > > > > > > > > ffff8880751d2b28
> > > > > > > > >  (
> > > > > > > > > &p->pi_lock
> > > > > > > > > ){-.-.}-{2:2}
> > > > > > > > > , at: try_to_wake_up+0x98/0x14a0 kernel/sched/core.c:3345
> > > > > > > > >  #1: 
> > > > > > > > > ffff8880b9d35258
> > > > > > > > >  (
> > > > > > > > > &rq->lock
> > > > > > > > > ){-.-.}-{2:2}
> > > > > > > > > , at: rq_lock kernel/sched/sched.h:1321 [inline]
> > > > > > > > > , at: ttwu_queue kernel/sched/core.c:3184 [inline]
> > > > > > > > > , at: try_to_wake_up+0x5e6/0x14a0 kernel/sched/core.c:3464
> > > > > > > > >  #2: ffff8880b9d1f948 (&per_cpu_ptr(group->pcpu, cpu)->seq){-.-.}-{0:0}, at: psi_task_change+0x142/0x220 kernel/sched/psi.c:807
> > > > > > > 
> > > > > > > This looks similar to syzbot+dde0cc33951735441301@syzkaller.appspotmail.com
> > > > > > > in that rcu_sleep_check() sees an RCU lock held, but the later call to
> > > > > > > lockdep_print_held_locks() does not.  Did something change recently that
> > > > > > > could let the ->lockdep_depth counter get out of sync with the actual
> > > > > > > number of locks held?
> > > > > > 
> > > > > > Dmitri had a different theory here:
> > > > > > 
> > > > > > https://groups.google.com/g/syzkaller-bugs/c/FmYvfZCZzqA/m/nc2CXUgsAgAJ
> > > > > 
> > > > > There is always room for more than one bug.  ;-)
> > > > > 
> > > > > He says "one-off false positives".  I was afraid of that...
> > > > 
> > > > And both the examples I have been copied on today are consistent with
> > > > debug_locks getting zeroed (e.g., via a call to __debug_locks_off())
> > > > in the midst of a call to rcu_sleep_check().  But I would expect to see
> > > > a panic or another splat if that were to happen.
> > > > 
> > > > Dmitry's example did have an additional splat, but I would expect the
> > > > RCU-related one to come second.  Again, there is always room for more
> > > > than one bug.
> > > > 
> > > > On the other hand, there are a lot more callers to debug_locks_off()
> > > > than there were last I looked into this.  And both of these splats
> > > > are consistent with an interrupt in the middle of rcu_sleep_check(),
> > > > and that interrupt's handler invoking debug_locks_off(), but without
> > > > printing anything to the console.  Does that sequence of events ring a
> > > > bell for anyone?
> > > > 
> > > > If this is the new normal, I could make RCU_LOCKDEP_WARN() recheck
> > > > debug_lockdep_rcu_enabled() after evaluating the condition, but with
> > > > a memory barrier immediately before the recheck.  But I am not at all
> > > > excited by doing this on speculation.  Especially given that doing
> > > > so might be covering up some other bug.
> > > > 
> > > 
> > > Just check the original console log and find:
> > > 
> > > [  356.696686][ T8418] =============================
> > > [  356.696692][ T8418] WARNING: suspicious RCU usage
> > > [  356.700193][T14782] ====================================
> > > [  356.704548][ T8418] 5.12.0-rc5-syzkaller #0 Not tainted
> > > [  356.729981][ T8418] -----------------------------
> > > [  356.732473][T14782] WARNING: iou-sqp-14780/14782 still has locks held!
> > > 
> > > , so there are two warnnings here, one is from lockdep_rcu_suspisous()
> > > and the other is from print_held_locks_bug(). I think this is what
> > > happened:
> > > 
> > > in RCU_LOCKDEP_WARN():
> > > 
> > > 	if (debug_lockdep_rcu_enabled() // this is true and at this time debug_locks = 1
> > > 	<interrupted>
> > > 	// lockdep detects a lock bug, set debug_locks = 0
> > > 	<swicth back>
> > > 	    && !__warned // true
> > > 	    && (c))      // "c" is a lock_is_held(), which will always returns true if debug_locks == 0!
> > > 
> > > the cause of the problem is that RCU_LOCKDEP_WARN() in fact read
> > > debug_locks twice and get different values.
> > > 
> > > But if you change the ordering of two reads, probably can avoid the
> > > problem:
> > > 	
> > > First read:
> > > 	lock_is_held(); // true if 1) lock is really held or 2) lockdep is off
> > > 
> > > Second read:
> > > 	debug_lockdep_rcu_enabled(); // if lockdep is not off, we know
> > > 				     // that the first read got correct
> > > 				     // value, otherwise we just ignore
> > > 				     // the first read, because either
> > > 				     // there is a bug reported between
> > > 				     // two reads, or lockdep is already
> > > 				     // off when the first read happens.
> > > 
> > > So maybe something below:
> > > 
> > > Regards,
> > > Boqun
> > > 
> > > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> > > index bd04f722714f..d11bee5d9347 100644
> > > --- a/include/linux/rcupdate.h
> > > +++ b/include/linux/rcupdate.h
> > > @@ -315,7 +315,7 @@ static inline int rcu_read_lock_any_held(void)
> > >  #define RCU_LOCKDEP_WARN(c, s)						\
> > >  	do {								\
> > >  		static bool __section(".data.unlikely") __warned;	\
> > > -		if (debug_lockdep_rcu_enabled() && !__warned && (c)) {	\
> > > +		if ((c) && debug_lockdep_rcu_enabled() && !__warned) {	\
> > >  			__warned = true;				\
> > >  			lockdep_rcu_suspicious(__FILE__, __LINE__, s);	\
> > >  		}							\
> > 
> > Good point -- if we check debug_lockdep_rcu_enabled() after the condition,
> > then we will reject false positives in cases where debug_locks was switched
> > to zero out from under us.
> > 
> > However, we do need ordering.  The "c" usually contains lock_is_held(),
> > which also checks debug_locks, but from some other translation unit.
> > Back in the day, the translation-unit boundaries would provide the needed
> > ordering, but LTO...
> > 
> > In addition, the "debug_locks = 0" was originally supposed to be a hint
> > that the report might be a false positive.  It is clear that this needs
> > to be made explicit.
> > 
> > Taking all this together, how about the following?  (The intent is
> > that the changes to lockdep_rcu_suspicious() will be in a separate
> > commit.)
> 
> Looks good to me ;-)

Whew!  May I add your Reviewed-by?

							Thanx, Paul

> Regards,
> Boqun
> 
> > 							Thanx, Paul
> > 
> > ------------------------------------------------------------------------
> > 
> > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> > index 9455476..1199ffd 100644
> > --- a/include/linux/rcupdate.h
> > +++ b/include/linux/rcupdate.h
> > @@ -315,7 +315,7 @@ static inline int rcu_read_lock_any_held(void)
> >  #define RCU_LOCKDEP_WARN(c, s)						\
> >  	do {								\
> >  		static bool __section(".data.unlikely") __warned;	\
> > -		if (debug_lockdep_rcu_enabled() && !__warned && (c)) {	\
> > +		if ((c) && debug_lockdep_rcu_enabled() && !__warned) {	\
> >  			__warned = true;				\
> >  			lockdep_rcu_suspicious(__FILE__, __LINE__, s);	\
> >  		}							\
> > diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> > index c6d0c1dc..80065cd 100644
> > --- a/kernel/locking/lockdep.c
> > +++ b/kernel/locking/lockdep.c
> > @@ -6387,6 +6387,7 @@ asmlinkage __visible void lockdep_sys_exit(void)
> >  void lockdep_rcu_suspicious(const char *file, const int line, const char *s)
> >  {
> >  	struct task_struct *curr = current;
> > +	int dl = READ_ONCE(debug_locks);
> >  
> >  	/* Note: the following can be executed concurrently, so be careful. */
> >  	pr_warn("\n");
> > @@ -6396,11 +6397,12 @@ void lockdep_rcu_suspicious(const char *file, const int line, const char *s)
> >  	pr_warn("-----------------------------\n");
> >  	pr_warn("%s:%d %s!\n", file, line, s);
> >  	pr_warn("\nother info that might help us debug this:\n\n");
> > -	pr_warn("\n%srcu_scheduler_active = %d, debug_locks = %d\n",
> > +	pr_warn("\n%srcu_scheduler_active = %d, debug_locks = %d\n%s",
> >  	       !rcu_lockdep_current_cpu_online()
> >  			? "RCU used illegally from offline CPU!\n"
> >  			: "",
> > -	       rcu_scheduler_active, debug_locks);
> > +	       rcu_scheduler_active, dl,
> > +	       dl ? "" : "Possible false positive due to lockdep disabling via debug_locks = 0\n");
> >  
> >  	/*
> >  	 * If a CPU is in the RCU-free window in idle (ie: in the section
> > diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
> > index b95ae86..dd94a60 100644
> > --- a/kernel/rcu/update.c
> > +++ b/kernel/rcu/update.c
> > @@ -277,7 +277,7 @@ EXPORT_SYMBOL_GPL(rcu_callback_map);
> >  
> >  noinstr int notrace debug_lockdep_rcu_enabled(void)
> >  {
> > -	return rcu_scheduler_active != RCU_SCHEDULER_INACTIVE && debug_locks &&
> > +	return rcu_scheduler_active != RCU_SCHEDULER_INACTIVE && READ_ONCE(debug_locks) &&
> >  	       current->lockdep_recursion == 0;
> >  }
> >  EXPORT_SYMBOL_GPL(debug_lockdep_rcu_enabled);

Re: [syzbot] WARNING: suspicious RCU usage in get_timespec64

[Boqun Feng]

On Mon, Apr 05, 2021 at 04:38:07PM -0700, Paul E. McKenney wrote:
> On Tue, Apr 06, 2021 at 07:25:44AM +0800, Boqun Feng wrote:
> > On Mon, Apr 05, 2021 at 10:27:52AM -0700, Paul E. McKenney wrote:
> > > On Mon, Apr 05, 2021 at 01:23:30PM +0800, Boqun Feng wrote:
> > > > On Sun, Apr 04, 2021 at 09:30:38PM -0700, Paul E. McKenney wrote:
> > > > > On Sun, Apr 04, 2021 at 09:01:25PM -0700, Paul E. McKenney wrote:
> > > > > > On Mon, Apr 05, 2021 at 04:08:55AM +0100, Matthew Wilcox wrote:
> > > > > > > On Sun, Apr 04, 2021 at 02:40:30PM -0700, Paul E. McKenney wrote:
> > > > > > > > On Sun, Apr 04, 2021 at 10:38:41PM +0200, Thomas Gleixner wrote:
> > > > > > > > > On Sun, Apr 04 2021 at 12:05, syzbot wrote:
> > > > > > > > > 
> > > > > > > > > Cc + ...
> > > > > > > > 
> > > > > > > > And a couple more...
> > > > > > > > 
> > > > > > > > > > Hello,
> > > > > > > > > >
> > > > > > > > > > syzbot found the following issue on:
> > > > > > > > > >
> > > > > > > > > > HEAD commit:    5e46d1b7 reiserfs: update reiserfs_xattrs_initialized() co..
> > > > > > > > > > git tree:       upstream
> > > > > > > > > > console output: https://syzkaller.appspot.com/x/log.txt?x=1125f831d00000
> > > > > > > > > > kernel config:  https://syzkaller.appspot.com/x/.config?x=78ef1d159159890
> > > > > > > > > > dashboard link: https://syzkaller.appspot.com/bug?extid=88e4f02896967fe1ab0d
> > > > > > > > > >
> > > > > > > > > > Unfortunately, I don't have any reproducer for this issue yet.
> > > > > > > > > >
> > > > > > > > > > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > > > > > > > > > Reported-by: syzbot+88e4f02896967fe1ab0d@syzkaller.appspotmail.com
> > > > > > > > > >
> > > > > > > > > > =============================
> > > > > > > > > > WARNING: suspicious RCU usage
> > > > > > > > > > 5.12.0-rc5-syzkaller #0 Not tainted
> > > > > > > > > > -----------------------------
> > > > > > > > > > kernel/sched/core.c:8294 Illegal context switch in RCU-sched read-side critical section!
> > > > > > > > > >
> > > > > > > > > > other info that might help us debug this:
> > > > > > > > > >
> > > > > > > > > >
> > > > > > > > > > rcu_scheduler_active = 2, debug_locks = 0
> > > > > > > > > > 3 locks held by syz-executor.4/8418:
> > > > > > > > > >  #0: 
> > > > > > > > > > ffff8880751d2b28
> > > > > > > > > >  (
> > > > > > > > > > &p->pi_lock
> > > > > > > > > > ){-.-.}-{2:2}
> > > > > > > > > > , at: try_to_wake_up+0x98/0x14a0 kernel/sched/core.c:3345
> > > > > > > > > >  #1: 
> > > > > > > > > > ffff8880b9d35258
> > > > > > > > > >  (
> > > > > > > > > > &rq->lock
> > > > > > > > > > ){-.-.}-{2:2}
> > > > > > > > > > , at: rq_lock kernel/sched/sched.h:1321 [inline]
> > > > > > > > > > , at: ttwu_queue kernel/sched/core.c:3184 [inline]
> > > > > > > > > > , at: try_to_wake_up+0x5e6/0x14a0 kernel/sched/core.c:3464
> > > > > > > > > >  #2: ffff8880b9d1f948 (&per_cpu_ptr(group->pcpu, cpu)->seq){-.-.}-{0:0}, at: psi_task_change+0x142/0x220 kernel/sched/psi.c:807
> > > > > > > > 
> > > > > > > > This looks similar to syzbot+dde0cc33951735441301@syzkaller.appspotmail.com
> > > > > > > > in that rcu_sleep_check() sees an RCU lock held, but the later call to
> > > > > > > > lockdep_print_held_locks() does not.  Did something change recently that
> > > > > > > > could let the ->lockdep_depth counter get out of sync with the actual
> > > > > > > > number of locks held?
> > > > > > > 
> > > > > > > Dmitri had a different theory here:
> > > > > > > 
> > > > > > > https://groups.google.com/g/syzkaller-bugs/c/FmYvfZCZzqA/m/nc2CXUgsAgAJ
> > > > > > 
> > > > > > There is always room for more than one bug.  ;-)
> > > > > > 
> > > > > > He says "one-off false positives".  I was afraid of that...
> > > > > 
> > > > > And both the examples I have been copied on today are consistent with
> > > > > debug_locks getting zeroed (e.g., via a call to __debug_locks_off())
> > > > > in the midst of a call to rcu_sleep_check().  But I would expect to see
> > > > > a panic or another splat if that were to happen.
> > > > > 
> > > > > Dmitry's example did have an additional splat, but I would expect the
> > > > > RCU-related one to come second.  Again, there is always room for more
> > > > > than one bug.
> > > > > 
> > > > > On the other hand, there are a lot more callers to debug_locks_off()
> > > > > than there were last I looked into this.  And both of these splats
> > > > > are consistent with an interrupt in the middle of rcu_sleep_check(),
> > > > > and that interrupt's handler invoking debug_locks_off(), but without
> > > > > printing anything to the console.  Does that sequence of events ring a
> > > > > bell for anyone?
> > > > > 
> > > > > If this is the new normal, I could make RCU_LOCKDEP_WARN() recheck
> > > > > debug_lockdep_rcu_enabled() after evaluating the condition, but with
> > > > > a memory barrier immediately before the recheck.  But I am not at all
> > > > > excited by doing this on speculation.  Especially given that doing
> > > > > so might be covering up some other bug.
> > > > > 
> > > > 
> > > > Just check the original console log and find:
> > > > 
> > > > [  356.696686][ T8418] =============================
> > > > [  356.696692][ T8418] WARNING: suspicious RCU usage
> > > > [  356.700193][T14782] ====================================
> > > > [  356.704548][ T8418] 5.12.0-rc5-syzkaller #0 Not tainted
> > > > [  356.729981][ T8418] -----------------------------
> > > > [  356.732473][T14782] WARNING: iou-sqp-14780/14782 still has locks held!
> > > > 
> > > > , so there are two warnnings here, one is from lockdep_rcu_suspisous()
> > > > and the other is from print_held_locks_bug(). I think this is what
> > > > happened:
> > > > 
> > > > in RCU_LOCKDEP_WARN():
> > > > 
> > > > 	if (debug_lockdep_rcu_enabled() // this is true and at this time debug_locks = 1
> > > > 	<interrupted>
> > > > 	// lockdep detects a lock bug, set debug_locks = 0
> > > > 	<swicth back>
> > > > 	    && !__warned // true
> > > > 	    && (c))      // "c" is a lock_is_held(), which will always returns true if debug_locks == 0!
> > > > 
> > > > the cause of the problem is that RCU_LOCKDEP_WARN() in fact read
> > > > debug_locks twice and get different values.
> > > > 
> > > > But if you change the ordering of two reads, probably can avoid the
> > > > problem:
> > > > 	
> > > > First read:
> > > > 	lock_is_held(); // true if 1) lock is really held or 2) lockdep is off
> > > > 
> > > > Second read:
> > > > 	debug_lockdep_rcu_enabled(); // if lockdep is not off, we know
> > > > 				     // that the first read got correct
> > > > 				     // value, otherwise we just ignore
> > > > 				     // the first read, because either
> > > > 				     // there is a bug reported between
> > > > 				     // two reads, or lockdep is already
> > > > 				     // off when the first read happens.
> > > > 
> > > > So maybe something below:
> > > > 
> > > > Regards,
> > > > Boqun
> > > > 
> > > > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> > > > index bd04f722714f..d11bee5d9347 100644
> > > > --- a/include/linux/rcupdate.h
> > > > +++ b/include/linux/rcupdate.h
> > > > @@ -315,7 +315,7 @@ static inline int rcu_read_lock_any_held(void)
> > > >  #define RCU_LOCKDEP_WARN(c, s)						\
> > > >  	do {								\
> > > >  		static bool __section(".data.unlikely") __warned;	\
> > > > -		if (debug_lockdep_rcu_enabled() && !__warned && (c)) {	\
> > > > +		if ((c) && debug_lockdep_rcu_enabled() && !__warned) {	\
> > > >  			__warned = true;				\
> > > >  			lockdep_rcu_suspicious(__FILE__, __LINE__, s);	\
> > > >  		}							\
> > > 
> > > Good point -- if we check debug_lockdep_rcu_enabled() after the condition,
> > > then we will reject false positives in cases where debug_locks was switched
> > > to zero out from under us.
> > > 
> > > However, we do need ordering.  The "c" usually contains lock_is_held(),
> > > which also checks debug_locks, but from some other translation unit.
> > > Back in the day, the translation-unit boundaries would provide the needed
> > > ordering, but LTO...
> > > 
> > > In addition, the "debug_locks = 0" was originally supposed to be a hint
> > > that the report might be a false positive.  It is clear that this needs
> > > to be made explicit.
> > > 
> > > Taking all this together, how about the following?  (The intent is
> > > that the changes to lockdep_rcu_suspicious() will be in a separate
> > > commit.)
> > 
> > Looks good to me ;-)
> 
> Whew!  May I add your Reviewed-by?
> 

Of course ;-)

Reviewed-by: Boqun Feng <boqun.feng@gmail.com>

Regards,
Boqun

> 							Thanx, Paul
> 
> > Regards,
> > Boqun
> > 
> > > 							Thanx, Paul
> > > 
> > > ------------------------------------------------------------------------
> > > 
> > > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> > > index 9455476..1199ffd 100644
> > > --- a/include/linux/rcupdate.h
> > > +++ b/include/linux/rcupdate.h
> > > @@ -315,7 +315,7 @@ static inline int rcu_read_lock_any_held(void)
> > >  #define RCU_LOCKDEP_WARN(c, s)						\
> > >  	do {								\
> > >  		static bool __section(".data.unlikely") __warned;	\
> > > -		if (debug_lockdep_rcu_enabled() && !__warned && (c)) {	\
> > > +		if ((c) && debug_lockdep_rcu_enabled() && !__warned) {	\
> > >  			__warned = true;				\
> > >  			lockdep_rcu_suspicious(__FILE__, __LINE__, s);	\
> > >  		}							\
> > > diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> > > index c6d0c1dc..80065cd 100644
> > > --- a/kernel/locking/lockdep.c
> > > +++ b/kernel/locking/lockdep.c
> > > @@ -6387,6 +6387,7 @@ asmlinkage __visible void lockdep_sys_exit(void)
> > >  void lockdep_rcu_suspicious(const char *file, const int line, const char *s)
> > >  {
> > >  	struct task_struct *curr = current;
> > > +	int dl = READ_ONCE(debug_locks);
> > >  
> > >  	/* Note: the following can be executed concurrently, so be careful. */
> > >  	pr_warn("\n");
> > > @@ -6396,11 +6397,12 @@ void lockdep_rcu_suspicious(const char *file, const int line, const char *s)
> > >  	pr_warn("-----------------------------\n");
> > >  	pr_warn("%s:%d %s!\n", file, line, s);
> > >  	pr_warn("\nother info that might help us debug this:\n\n");
> > > -	pr_warn("\n%srcu_scheduler_active = %d, debug_locks = %d\n",
> > > +	pr_warn("\n%srcu_scheduler_active = %d, debug_locks = %d\n%s",
> > >  	       !rcu_lockdep_current_cpu_online()
> > >  			? "RCU used illegally from offline CPU!\n"
> > >  			: "",
> > > -	       rcu_scheduler_active, debug_locks);
> > > +	       rcu_scheduler_active, dl,
> > > +	       dl ? "" : "Possible false positive due to lockdep disabling via debug_locks = 0\n");
> > >  
> > >  	/*
> > >  	 * If a CPU is in the RCU-free window in idle (ie: in the section
> > > diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
> > > index b95ae86..dd94a60 100644
> > > --- a/kernel/rcu/update.c
> > > +++ b/kernel/rcu/update.c
> > > @@ -277,7 +277,7 @@ EXPORT_SYMBOL_GPL(rcu_callback_map);
> > >  
> > >  noinstr int notrace debug_lockdep_rcu_enabled(void)
> > >  {
> > > -	return rcu_scheduler_active != RCU_SCHEDULER_INACTIVE && debug_locks &&
> > > +	return rcu_scheduler_active != RCU_SCHEDULER_INACTIVE && READ_ONCE(debug_locks) &&
> > >  	       current->lockdep_recursion == 0;
> > >  }
> > >  EXPORT_SYMBOL_GPL(debug_lockdep_rcu_enabled);