Re: [PATCH v2] rcu: Fix missed wakeup of exp_wq waiters

[Paul E. McKenney]

On Wed, Nov 20, 2019 at 10:28:38AM +0530, Neeraj Upadhyay wrote:
> 
> On 11/20/2019 1:08 AM, Paul E. McKenney wrote:
> > On Tue, Nov 19, 2019 at 03:17:07AM +0000, 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(). This task blocks at wq index 1.
> > > 
> > >     synchronize_rcu_expedited()
> > >       s = 0b'100'
> > >       exp_funnel_lock()
> > >         wait_event(rnp->exp_wq[rcu_seq_ctr(s) & 0x3]
> > > 
> > > 2. The expedited grace period (which above task blocks for)
> > >     completes and task (task1) holding exp_mutex queues
> > >     worker and schedules out.
> > > 
> > >     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 and then blocks on exp_wake_mutex, which is held
> > >     by another kworker, which is doing wakeups for expedited_sequence
> > >     0.
> > > 
> > >     rcu_exp_wait_wake()
> > >       rcu_exp_wait_wake()
> > >         rcu_exp_gp_seq_end(rsp) // rsp->expedited_sequence is incremented
> > >                                 // to 0b'100'
> > >         mutex_lock(&rcu_state.exp_wake_mutex)
> > > 
> > > 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);
> > > 
> > > 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. kworker A acquires exp_wake_mutex. As it 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]);
> > > 
> > > This issue manifested as DPM device timeout during suspend, as scsi
> > > device was stuck in _synchronize_rcu_expedited().
> > > 
> > > schedule()
> > > synchronize_rcu_expedited()
> > > synchronize_rcu()
> > > scsi_device_quiesce()
> > > scsi_bus_suspend()
> > > dpm_run_callback()
> > > __device_suspend()
> > > 
> > > Fix this by using the correct exp sequence number, the one which
> > > owner of the exp_mutex initiated and passed to kworker,
> > > to index the wait queue, inside rcu_exp_wait_wake().
> > > 
> > > Signed-off-by: Neeraj Upadhyay <neeraju@xxxxxxxxxxxxxx>
> > 
> > Queued, thank you!
> > 
> > I reworked the commit message to make it easier to follow the sequence
> > of events.  Please see below and let me know if I messed anything up.
> > 
> > 							Thanx, Paul
> > 
> > ------------------------------------------------------------------------
> > 
> > commit d887fd2a66861f51ed93b5dde894b9646a5569dd
> > Author: Neeraj Upadhyay <neeraju@xxxxxxxxxxxxxx>
> > Date:   Tue Nov 19 03:17:07 2019 +0000
> > 
> >      rcu: Fix missed wakeup of exp_wq waiters
> >      Tasks waiting within exp_funnel_lock() for an expedited grace period to
> >      elapse can be starved due to the following sequence of events:
> >      1.      Tasks A and B both attempt to start an expedited grace
> >              period at about the same time.  This grace period will have
> >              completed when the lower four bits of the rcu_state structure's
> >              ->expedited_sequence field are 0b'0100', for example, when the
> >              initial value of this counter is zero.  Task A wins, and thus
> >              does the actual work of starting the grace period, including
> >              acquiring the rcu_state structure's .exp_mutex and sets the
> >              counter to 0b'0001'.
> >      2.      Because task B lost the race to start the grace period, it
> >              waits on ->expedited_sequence to reach 0b'0100' inside of
> >              exp_funnel_lock(). This task therefore blocks on the rcu_node
> >              structure's ->exp_wq[1] field, keeping in mind that the
> >              end-of-grace-period value of ->expedited_sequence (0b'0100')
> >              is shifted down two bits before indexing the ->exp_wq[] field.
> >      3.      Task C attempts to start another expedited grace period,
> >              but blocks on ->exp_mutex, which is still held by Task A.
> >      4.      The aforementioned expedited grace period completes, so that
> >              ->expedited_sequence now has the value 0b'0100'.  A kworker task
> >              therefore acquires the rcu_state structure's ->exp_wake_mutex
> >              and starts awakening any tasks waiting for this grace period.
> >      5.      One of the first tasks awakened happens to be Task A.  Task A
> >              therefore releases the rcu_state structure's ->exp_mutex,
> >              which allows Task C to start the next expedited grace period,
> >              which causes the lower four bits of the rcu_state structure's
> >              ->expedited_sequence field to become 0b'0101'.
> >      6.      Task C's expedited grace period completes, so that the lower four
> >              bits of the rcu_state structure's ->expedited_sequence field now
> >              become 0b'1000'.
> >      7.      The kworker task from step 4 above continues its wakeups.
> >              Unfortunately, the wake_up_all() refetches the rcu_state
> >              structure's .expedited_sequence field:
> 
> This might not be true. I think wake_up_all(), which internally calls
> __wake_up(), will use a single wq_head for doing all wakeups. So, a single
> .expedited_sequence value would be used to get wq_head.
> 
> void __wake_up(struct wait_queue_head *wq_head, ...)

The wake_up_all() really is selecting among four different ->exp_wq[]
array entries:

wake_up_all(&rnp->exp_wq[rcu_seq_ctr(rcu_state.expedited_sequence) & 0x3]);

So I lost you here.  Are you saying that the wake_up_all() will somehow
be operating on ->exp_wq[1], which is where Task B is blocked?  Or that
Task B would instead be blocked on ->exp_wq[2]?  Or that failing to wake
up Task B is OK for some reason?  Or something else entirely?

							Thanx, Paul

> However, below sequence of events would result in problem:
> 
> 1.      Tasks A starts an expedited grace period at about the same time.
>         This grace period will have completed when the lower four bits
> 		of the rcu_state structure's ->expedited_sequence field are 0b'0100',
> 		for example, when the initial value of this counter is zero.
> 		Task A wins, acquires the rcu_state structure's .exp_mutex and
> 		sets the counter to 0b'0001'.
> 
> 2.      The aforementioned expedited grace period completes, so that
>         ->expedited_sequence now has the value 0b'0100'.  A kworker task
>         therefore acquires the rcu_state structure's ->exp_wake_mutex
>         and starts awakening any tasks waiting for this grace period.
>         This kworker gets preempted while unlocking wq_head lock
> 
>         wake_up_all()
>           __wake_up()
>             __wake_up_common_lock()
>               spin_unlock_irqrestore()
>                 __raw_spin_unlock_irqrestore()
>                   preempt_enable()
>                     __preempt_schedule()
> 
> 3.      One of the first tasks awakened happens to be Task A.  Task A
>         therefore releases the rcu_state structure's ->exp_mutex,
> 
> 4.      Tasks B and C both attempt to start an expedited grace
>         period at about the same time.  This grace period will have
>         completed when the lower four bits of the rcu_state structure's
>         ->expedited_sequence field are 0b'1000'. Task B wins, and thus
>         does the actual work of starting the grace period, including
>         acquiring the rcu_state structure's .exp_mutex and sets the
>         counter to 0b'0101'.
> 
> 5.      Because task C lost the race to start the grace period, it
>         waits on ->expedited_sequence to reach 0b'1000' inside of
>         exp_funnel_lock(). This task therefore blocks on the rcu_node
>         structure's ->exp_wq[2] field, keeping in mind that the
>         end-of-grace-period value of ->expedited_sequence (0b'1000')
>         is shifted down two bits before indexing the ->exp_wq[] field.
> 
> 6.      Task B queues work to complete expedited grace period. This
>         task is preempted just before wait_event call. Kworker task picks
> 		up the work queued by task B and and completes grace period, so
> 		that the lower four bits of the rcu_state structure's
> 		->expedited_sequence field now become 0b'1000'. This kworker starts
> 		waiting on the exp_wake_mutex, which is owned by kworker doing
> 		wakeups for expedited sequence initiated by task A.
> 
> 7.      Task B schedules in and finds its expedited sequence snapshot has
>         completed; so, it does not enter waitq and releases exp_mutex. This
> 		allows Task D to start the next expedited grace period,
>         which causes the lower four bits of the rcu_state structure's
>         ->expedited_sequence field to become 0b'1001'.
> 
> 8.      Task D's expedited grace period completes, so that the lower four
>         bits of the rcu_state structure's ->expedited_sequence field now
>         become 0b'1100'.
> 
> 9.      kworker from step 2 is scheduled in and releases exp_wake_mutex;
>         kworker correspnding to Task B's expedited grace period acquires
> 		exp_wake_mutex and starts wakeups. Unfortunately, it used the
> 		rcu_state structure's .expedited_sequence field for determining
> 		the waitq index.
> 
> 
> wake_up_all(&rnp->exp_wq[rcu_seq_ctr(rcu_state.expedited_sequence) & 0x3]);
> 
>         This results in the wakeup being applied to the rcu_node
>         structure's ->exp_wq[3] field, which is unfortunate given that
>         Task C is instead waiting on ->exp_wq[2].
> 
> 
> >              wake_up_all(&rnp->exp_wq[rcu_seq_ctr(rcu_state.expedited_sequence) & 0x3]);
> >              This results in the wakeup being applied to the rcu_node
> >              structure's ->exp_wq[2] field, which is unfortunate given that
> >              Task B is instead waiting on ->exp_wq[1].
> >      On a busy system, no harm is done (or at least no permanent harm is done).
> >      Some later expedited grace period will redo the wakeup.  But on a quiet
> >      system, such as many embedded systems, it might be a good long time before
> >      there was another expedited grace period.  On such embedded systems,
> >      this situation could therefore result in a system hang.
> >      This issue manifested as DPM device timeout during suspend (which
> >      usually qualifies as a quiet time) due to a SCSI device being stuck in
> >      _synchronize_rcu_expedited(), with the following stack trace:
> >              schedule()
> >              synchronize_rcu_expedited()
> >              synchronize_rcu()
> >              scsi_device_quiesce()
> >              scsi_bus_suspend()
> >              dpm_run_callback()
> >              __device_suspend()
> >      This commit therefore prevents such delays, timeouts, and hangs by
> >      making rcu_exp_wait_wake() use its "s" argument consistently instead of
> >      refetching from rcu_state.expedited_sequence.
> 
> Do we need a "fixes" tag here?
> 
> >      Signed-off-by: Neeraj Upadhyay <neeraju@xxxxxxxxxxxxxx>
> >      Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx
> > 
> > diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> > index 6ce598d..4433d00a 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);
> > 
> 
> -- 
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