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Date: Wed, 20 Nov 2019 20:07:51 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Neeraj Upadhyay <neeraju@...eaurora.org>
Cc: josh@...htriplett.org, rostedt@...dmis.org,
mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
joel@...lfernandes.org, linux-kernel@...r.kernel.org,
gkohli@...eaurora.org, prsood@...eaurora.org,
pkondeti@...eaurora.org, rcu@...r.kernel.org
Subject: Re: [PATCH v2] rcu: Fix missed wakeup of exp_wq waiters
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@...eaurora.org>
> >
> > 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@...eaurora.org>
> > 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@...eaurora.org>
> > Signed-off-by: Paul E. McKenney <paulmck@...nel.org
> >
> > 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);
> >
>
> --
> QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member of
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