Re: [syzbot] [kernel?] KCSAN: assert: race in dequeue_entities

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Oct 22, 2024 at 10:06:23AM +0200, Alexander Potapenko wrote:
> On Fri, Sep 27, 2024 at 4:57 PM syzbot
> <syzbot+0ec1e96c2cdf5c0e512a@syzkaller.appspotmail.com> wrote:
> >
> > Hello,
> >
> > syzbot found the following issue on:
> >
> > HEAD commit:    075dbe9f6e3c Merge tag 'soc-ep93xx-dt-6.12' of git://git.k..
> > git tree:       upstream
> > console output: https://syzkaller.appspot.com/x/log.txt?x=15f07a80580000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=86254f9e0a8f2c98
> > dashboard link: https://syzkaller.appspot.com/bug?extid=0ec1e96c2cdf5c0e512a
> > compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
> >
> > Unfortunately, I don't have any reproducer for this issue yet.
> >
> > Downloadable assets:
> > disk image: https://storage.googleapis.com/syzbot-assets/1be80941df60/disk-075dbe9f.raw.xz
> > vmlinux: https://storage.googleapis.com/syzbot-assets/494a9ac89c09/vmlinux-075dbe9f.xz
> > kernel image: https://storage.googleapis.com/syzbot-assets/919788d8c731/bzImage-075dbe9f.xz
> >
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+0ec1e96c2cdf5c0e512a@syzkaller.appspotmail.com
> >
> > audit: type=1400 audit(6022412112.141:243086): avc:  denied  { read } for  pid=2950 comm="syslogd" name="log" dev="sda1" ino=1915 scontext=system_u:system_r:syslogd_t tcontext=system_u:object_r:var_t tclass=lnk_file permissive=0
> > audit: type=1400 audit(6022412112.181:243087): avc:  denied  { read } for  pid=2950 comm="syslogd" name="log" dev="sda1" ino=1915 scontext=system_u:system_r:syslogd_t tcontext=system_u:object_r:var_t tclass=lnk_file permissive=0
> > ==================================================================
> > BUG: KCSAN: assert: race in __block_task kernel/sched/sched.h:2770 [inline]
> > BUG: KCSAN: assert: race in dequeue_entities+0x6df/0x760 kernel/sched/fair.c:7177
> >
> > race at unknown origin, with assert no writes to 0xffff888101764268 of 4 bytes by task 29 on cpu 1:
> >  __block_task kernel/sched/sched.h:2770 [inline]
> >  dequeue_entities+0x6df/0x760 kernel/sched/fair.c:7177
> >  pick_next_entity kernel/sched/fair.c:5627 [inline]
> >  pick_task_fair kernel/sched/fair.c:8856 [inline]
> >  pick_next_task_fair+0x7d/0x410 kernel/sched/fair.c:8876
> >  __pick_next_task kernel/sched/core.c:5955 [inline]
> >  pick_next_task kernel/sched/core.c:6477 [inline]
> >  __schedule+0x284/0x940 kernel/sched/core.c:6629
> >  __schedule_loop kernel/sched/core.c:6752 [inline]
> >  schedule+0x55/0xc0 kernel/sched/core.c:6767
> >  kauditd_thread+0x56b/0x650 kernel/audit.c:911
> >  kthread+0x1d1/0x210 kernel/kthread.c:389
> >  ret_from_fork+0x4b/0x60 arch/x86/kernel/process.c:147
> >  ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244
> >
> > value changed: 0x00000000 -> 0x00000001
> >
> > Reported by Kernel Concurrency Sanitizer on:
> > CPU: 1 UID: 0 PID: 29 Comm: kauditd Not tainted 6.11.0-syzkaller-11558-g075dbe9f6e3c #0
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/13/2024
> > ==================================================================
> 
> +PeterZ, who added the KCSAN assertion.

Well, PaulMck did in d6111cf45c57 ("sched: Use WRITE_ONCE() for
p->on_rq"), I just moved it around in e8901061ca0c ("sched: Split
DEQUEUE_SLEEP from deactivate_task()").

I'm not at all sure I have any inkling as to what the annotation does
nor what KCSAN is trying to tell us above.

Can someone please translate?