Re: rcu: WARNING in rcu_seq_end

[Paul E. McKenney]

On Tue, Mar 07, 2017 at 08:05:19AM +0100, Dmitry Vyukov wrote:
> On Tue, Mar 7, 2017 at 12:08 AM, Paul E. McKenney
> <paulmck@xxxxxxxxxxxxxxxxxx> 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
> >> >>
> >> >> Ah, you meant to synchronize rcu_seq_end with rcu_seq_done?
> >> >
> >> > No, there is a mutex release and acquisition that do the synchronization,
> >> > but only if the wakeup has appropriate barriers.  The issue is that
> >> > part of the mutex's critical section executes in a workqueue, possibly
> >> > on some other CPU.
> >>
> >> What is that mutex? And what locks/unlocks provide synchronization? I
> >> see that one uses exp_mutex and another -- exp_wake_mutex.
> >
> > Both of them.
> >
> > ->exp_mutex is acquired by the task requesting the grace period, and
> > the counter's first increment is done by that task under that mutex.
> > This task then schedules a workqueue, which drives forward the grace
> > period.  Upon grace-period completion, the workqueue handler does the
> > second increment (the one that your patch addressed).  The workqueue
> > handler then acquires ->exp_wake_mutex and wakes the task that holds
> > ->exp_mutex (along with all other tasks waiting for this grace period),
> > and that task releases ->exp_mutex, which allows the next grace period to
> > start (and the first increment for that next grace period to be carried
> > out under that lock).  The workqueue handler releases ->exp_wake_mutex
> > after finishing its wakeups.

First and foremost, thank you for reviewing this stuff!!!

> Then we need the following for the case when task requesting the grace
> period does not block, right?
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index d80c2587bed8..aa7ba83f6a56 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -3534,7 +3534,7 @@ static void rcu_seq_start(unsigned long *sp)
>  static void rcu_seq_end(unsigned long *sp)
>  {
>         smp_mb(); /* Ensure update-side operation before counter increment. */
> -       WRITE_ONCE(*sp, *sp + 1);
> +       smp_store_release(sp, *sp + 1);
>         WARN_ON_ONCE(*sp & 0x1);
>  }

The above is not needed, as the smp_mb() covers it completely.

> @@ -3554,7 +3554,7 @@ static unsigned long rcu_seq_snap(unsigned long *sp)
>   */
>  static bool rcu_seq_done(unsigned long *sp, unsigned long s)
>  {
> -       return ULONG_CMP_GE(READ_ONCE(*sp), s);
> +       return ULONG_CMP_GE(smp_load_acquire(sp), s);

And this one is covered by the smp_mb__before_atomic() inside the "if"
statement in sync_exp_work_done(), via rcu_exp_gp_seq_done().  That is,
sync_exp_work_done() invokes rcu_exp_gp_seq_done() which in turn invokes
rcu_seq_done().

Why smp_mb() instead of release-acquire?  Because the grace-period
relationship is quite strong, it cannot be implemented with
release-acquire on all platforms.

							Thanx, Paul