Re: [PATCH] rcu: Fix missed wakeup of exp_wq waiters

["Paul E. McKenney" <paulmck@kernel.org>]

On Mon, Nov 18, 2019 at 09:28:39AM +0000, Neeraj Upadhyay wrote:
> Hi Paul,
> 
> On 11/18/2019 3:06 AM, Paul E. McKenney wrote:
> > On Fri, Nov 15, 2019 at 10:58:14PM +0530, Neeraj Upadhyay wrote:
> > > For the tasks waiting in exp_wq inside exp_funnel_lock(),
> > > there is a chance that they might be indefinitely blocked
> > > in below scenario:
> > > 
> > > 1. There is a task waiting on exp sequence 0b'100' inside
> > >     exp_funnel_lock().
> > > 
> > >     _synchronize_rcu_expedited()
> > 
> > This symbol went away a few versions back, but let's see how this
> > plays out in current -rcu.
> > 
> 
> Sorry; for us this problem is observed on 4.19 stable version; I had
> checked against the -rcu code, and the relevant portions were present
> there.
> 
> > >       s = 0b'100
> > >       exp_funnel_lock()
> > >         wait_event(rnp->exp_wq[rcu_seq_ctr(s) & 0x3]
> > 
> > All of the above could still happen if the expedited grace
> > period number was zero (or a bit less) when that task invoked
> 
> Yes
> 
> > synchronize_rcu_expedited().  What is the relation, if any,
> > between this task and "task1" below?  Seems like you want them to
> > be different tasks.
> > 
> 
> This task is the one which is waiting for the expedited sequence, which
> "task1" completes ("task1" holds the exp_mutex for it). "task1" would
> wake up this task, on exp GP completion.
> 
> > Does this task actually block, or is it just getting ready
> > to block?  Seems like you need it to have actually blocked.
> > 
> 
> Yes, it actually blocked in wait queue.
> 
> > > 2. The Exp GP completes and task (task1) holding exp_mutex queues
> > >     worker and schedules out.
> > 
> > "The Exp GP" being the one that was initiated when the .expedited_sequence
> > counter was zero, correct?  (Looks that way below.)
> > 
> Yes, correct.
> 
> > >     _synchronize_rcu_expedited()
> > >       s = 0b'100
> > >       queue_work(rcu_gp_wq, &rew.rew_work)
> > >         wake_up_worker()
> > >           schedule()
> > > 
> > > 3. kworker A picks up the queued work and completes the exp gp
> > >     sequence.
> > > 
> > >     rcu_exp_wait_wake()
> > >       rcu_exp_wait_wake()
> > >         rcu_exp_gp_seq_end(rsp) // rsp->expedited_sequence is incremented
> > >                                 // to 0b'100'
> > > 
> > > 4. task1 does not enter wait queue, as sync_exp_work_done() returns true,
> > >     and releases exp_mutex.
> > > 
> > >     wait_event(rnp->exp_wq[rcu_seq_ctr(s) & 0x3],
> > >       sync_exp_work_done(rsp, s));
> > >     mutex_unlock(&rsp->exp_mutex);
> > 
> > So task1 is the one that initiated the expedited grace period that
> > started when .expedited_sequence was zero, right?
> > 
> 
> Yes, right.
> 
> > > 5. Next exp GP completes, and sequence number is incremented:
> > > 
> > >     rcu_exp_wait_wake()
> > >       rcu_exp_wait_wake()
> > >         rcu_exp_gp_seq_end(rsp) // rsp->expedited_sequence = 0b'200'
> > > 
> > > 6. As kworker A uses current expedited_sequence, it wakes up workers
> > >     from wrong wait queue index - it should have worken wait queue
> > >     corresponding to 0b'100' sequence, but wakes up the ones for
> > >     0b'200' sequence. This results in task at step 1 indefinitely blocked.
> > > 
> > >     rcu_exp_wait_wake()
> > >       wake_up_all(&rnp->exp_wq[rcu_seq_ctr(rsp->expedited_sequence) & 0x3]);
> > 
> > So the issue is that the next expedited RCU grace period might
> > have completed before the completion of the wakeups for the previous
> > expedited RCU grace period, correct?  Then expedited grace periods have
> 
> Yes. Actually from the ftraces, I saw that next expedited RCU grace
> period completed while kworker A was in D state, while waiting for
> exp_wake_mutex. This led to kworker A using sequence 2 (instead of 1) for
> its wake_up_all() call; so, task (point 1) was never woken up, as it was
> waiting on wq index 1.
> 
> > to have stopped to prevent any future wakeup from happening, correct?
> > (Which would make it harder for rcutorture to trigger this, though it
> > really does have code that attempts to trigger this sort of thing.)
> > 
> > Is this theoretical in nature, or have you actually triggered it?
> > If actually triggered, what did you do to make this happen?
> 
> This issue, we had seen previously - 1 instance in May 2018 (on 4.9 kernel),
> another instance in Nov 2018 (on 4.14 kernel), in our customer reported
> issues. Both instances were in downstream drivers and we didn't have RCU
> traces. Now 2 days back, it was reported on 4.19 kernel, with RCU traces
> enabled, where it was observed in suspend scenario, where we are observing
> "DPM device timeout" [1], as scsi device is stuck in
> _synchronize_rcu_expedited().
> 
> schedule+0x70/0x90
> _synchronize_rcu_expedited+0x590/0x5f8
> synchronize_rcu+0x50/0xa0
> scsi_device_quiesce+0x50/0x120
> scsi_bus_suspend+0x70/0xe8
> dpm_run_callback+0x148/0x388
> __device_suspend+0x430/0x8a8
> 
> [1]
> https://github.com/torvalds/linux/blob/master/drivers/base/power/main.c#L489
> 
> > What have you done to test the change?
> > 
> 
> I have given this for testing; will share the results . Current analysis
> and patch is based on going through ftrace and code review.

OK, very good.  Please include the failure information in the changelog
of the next version of this patch.

I prefer your original patch, that just uses "s", over the one below
that moves the rcu_exp_gp_seq_end().  The big advantage of your original
patch is that it allow more concurrency between a consecutive pair of
expedited RCU grace periods.  Plus it would not be easy to convince
myself that moving rcu_exp_gp_seq_end() down is safe, so your original
is also conceptually simpler with a more manageable state space.

Please also add the WARN_ON(), though at first glance your change seems
to have lost the wakeup.  (But it is early, so maybe it is just that I
am not yet fully awake.)

							Thanx, Paul

> I was thinking of another way of addressing this problem: Doing exp seq end
> inside exp_wake_mutex. This will also ensure that, if we extend the current
> scenario and there are multiple expedited GP sequence, which have completed,
> before exp_wake_mutex is held, we need to preserve the requirement of 3 wq
> entries [2].
> 
> 
> [2] https://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git/tree/Documentation/RCU/Design/Expedited-Grace-Periods/Expedited-Grace-Periods.rst?h=dev
> 
> 
> @@ -595,8 +595,6 @@ static void rcu_exp_wait_wake(struct rcu_state *rsp,
> unsigned long s)
>         struct rcu_node *rnp;
> 
>         synchronize_sched_expedited_wait(rsp);
> -       rcu_exp_gp_seq_end(rsp);
> -       trace_rcu_exp_grace_period(rsp->name, s, TPS("end"));
> 
>         /*
>          * Switch over to wakeup mode, allowing the next GP, but -only- the
> @@ -604,6 +602,9 @@ static void rcu_exp_wait_wake(struct rcu_state *rsp,
> unsigned long s)
>          */
>         mutex_lock(&rsp->exp_wake_mutex);
> 
> +       rcu_exp_gp_seq_end(rsp);
> +       trace_rcu_exp_grace_period(rsp->name, s, TPS("end"));
> +
> 
> 
> 
> > (Using a WARN_ON() to check for the lower bits of the counter portion
> > of rcu_state.expedited_sequence differing from the same bits of s
> > would be one way to detect this problem.)
> > 
> > 							Thanx, Paul
> > 
> 
> I have also given the patch for this, for testing:
> 
>  static void rcu_exp_wait_wake(struct rcu_state *rsp, unsigned long s)
>  {
>         struct rcu_node *rnp;
> +       unsigned long exp_low;
> +       unsigned long s_low = rcu_seq_ctr(s) & 0x3;
> 
>         synchronize_sched_expedited_wait(rsp);
>         rcu_exp_gp_seq_end(rsp);
> @@ -613,7 +615,9 @@ static void rcu_exp_wait_wake(struct rcu_state *rsp,
> unsigned long s)
>                         spin_unlock(&rnp->exp_lock);
>                 }
>                 smp_mb(); /* All above changes before wakeup. */
> - wake_up_all(&rnp->exp_wq[rcu_seq_ctr(rsp->expedited_sequence) & 0x3]);
> +               exp_low = rcu_seq_ctr(rsp->expedited_sequence) & 0x3;
> +               WARN_ON(s_low != exp_low);
> +
> 
> Thanks
> Neeraj
> 
> > > Fix this by using the correct sequence number for wake_up_all() inside
> > > rcu_exp_wait_wake().
> > > 
> > > Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org>
> > > ---
> > >   kernel/rcu/tree_exp.h | 2 +-
> > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> > > index e4b77d3..28979d3 100644
> > > --- a/kernel/rcu/tree_exp.h
> > > +++ b/kernel/rcu/tree_exp.h
> > > @@ -557,7 +557,7 @@ static void rcu_exp_wait_wake(unsigned long s)
> > >   			spin_unlock(&rnp->exp_lock);
> > >   		}
> > >   		smp_mb(); /* All above changes before wakeup. */
> > > -		wake_up_all(&rnp->exp_wq[rcu_seq_ctr(rcu_state.expedited_sequence) & 0x3]);
> > > +		wake_up_all(&rnp->exp_wq[rcu_seq_ctr(s) & 0x3]);
> > >   	}
> > >   	trace_rcu_exp_grace_period(rcu_state.name, s, TPS("endwake"));
> > >   	mutex_unlock(&rcu_state.exp_wake_mutex);
> > > -- 
> > > QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a
> > > member of the Code Aurora Forum, hosted by The Linux Foundation
> > > 
> 
> -- 
> QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member of
> the Code Aurora Forum, hosted by The Linux Foundation