Re: rcu: WARNING in rcu_seq_end

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Sun, Mar 05, 2017 at 11:50:39AM +0100, Dmitry Vyukov wrote:
> On Sat, Mar 4, 2017 at 9:40 PM, Paul E. McKenney
> <paulmck@linux.vnet.ibm.com> wrote:
> > On Sat, Mar 04, 2017 at 05:01:19PM +0100, Dmitry Vyukov wrote:
> >> Hello,
> >>
> >> Paul, you wanted bugs in rcu.
> >
> > Well, whether I want them or not, I must deal with them.  ;-)
> >
> >> I've got this WARNING while running syzkaller fuzzer on
> >> 86292b33d4b79ee03e2f43ea0381ef85f077c760:
> >>
> >> ------------[ cut here ]------------
> >> WARNING: CPU: 0 PID: 4832 at kernel/rcu/tree.c:3533
> >> rcu_seq_end+0x110/0x140 kernel/rcu/tree.c:3533
> >> Kernel panic - not syncing: panic_on_warn set ...
> >> CPU: 0 PID: 4832 Comm: kworker/0:3 Not tainted 4.10.0+ #276
> >> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
> >> Workqueue: events wait_rcu_exp_gp
> >> Call Trace:
> >>  __dump_stack lib/dump_stack.c:15 [inline]
> >>  dump_stack+0x2ee/0x3ef lib/dump_stack.c:51
> >>  panic+0x1fb/0x412 kernel/panic.c:179
> >>  __warn+0x1c4/0x1e0 kernel/panic.c:540
> >>  warn_slowpath_null+0x2c/0x40 kernel/panic.c:583
> >>  rcu_seq_end+0x110/0x140 kernel/rcu/tree.c:3533
> >>  rcu_exp_gp_seq_end kernel/rcu/tree_exp.h:36 [inline]
> >>  rcu_exp_wait_wake+0x8a9/0x1330 kernel/rcu/tree_exp.h:517
> >>  rcu_exp_sel_wait_wake kernel/rcu/tree_exp.h:559 [inline]
> >>  wait_rcu_exp_gp+0x83/0xc0 kernel/rcu/tree_exp.h:570
> >>  process_one_work+0xc06/0x1c20 kernel/workqueue.c:2096
> >>  worker_thread+0x223/0x19c0 kernel/workqueue.c:2230
> >>  kthread+0x326/0x3f0 kernel/kthread.c:227
> >>  ret_from_fork+0x31/0x40 arch/x86/entry/entry_64.S:430
> >> Dumping ftrace buffer:
> >>    (ftrace buffer empty)
> >> Kernel Offset: disabled
> >> Rebooting in 86400 seconds..
> >>
> >>
> >> Not reproducible. But looking at the code, shouldn't it be:
> >>
> >>  static void rcu_seq_end(unsigned long *sp)
> >>  {
> >>         smp_mb(); /* Ensure update-side operation before counter increment. */
> >> +       WARN_ON_ONCE(!(*sp & 0x1));
> >>         WRITE_ONCE(*sp, *sp + 1);
> >> -       WARN_ON_ONCE(*sp & 0x1);
> >>  }
> >>
> >> ?
> >>
> >> Otherwise wait_event in _synchronize_rcu_expedited can return as soon
> >> as WRITE_ONCE(*sp, *sp + 1) finishes. As far as I understand this
> >> consequently can allow start of next grace periods. Which in turn can
> >> make the warning fire. Am I missing something?
> >>
> >> I don't see any other bad consequences of this. The rest of
> >> rcu_exp_wait_wake can proceed when _synchronize_rcu_expedited has
> >> returned and destroyed work on stack and next period has started and
> >> ended, but it seems OK.
> >
> > I believe that this is a heygood change, but I don't see how it will
> > help in this case.  BTW, may I have your Signed-off-by?
> >
> > The reason I don't believe that it will help is that the
> > rcu_exp_gp_seq_end() function is called from a workqueue handler that
> > is invoked holding ->exp_mutex, and this mutex is not released until
> > after the handler invokes rcu_seq_end() and then wakes up the task that
> > scheduled the workqueue handler.  So the ordering above should not matter
> > (but I agree that your ordering is cleaner.
> >
> > That said, it looks like I am missing some memory barriers, please
> > see the following patch.
> >
> > But what architecture did you see this on?
> 
> 
> This is just x86.
> 
> You seem to assume that wait_event() waits for the wakeup. It does not
> work this way. It can return as soon as the condition becomes true
> without ever waiting:
> 
> 305 #define wait_event(wq, condition)                                       \
> 306 do {                                                                    \
> 307         might_sleep();                                                  \
> 308         if (condition)                                                  \
> 309                 break;                                                  \
> 310         __wait_event(wq, condition);                                    \
> 311 } while (0)

Agreed, hence my patch in the previous email.  I guess I knew that, but
on the day I wrote that code, my fingers didn't.  Or somew similar lame
excuse.  ;-)

> Mailed a signed patch:
> https://groups.google.com/d/msg/syzkaller/XzUXuAzKkCw/5054wU9MEAAJ

This is the patch you also sent by email, that moves the WARN_ON_ONCE(),
thank you!

							Thanx, Paul
>