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Date: Wed, 20 Nov 2019 04:58:46 +0000
From: Neeraj Upadhyay <neeraju@...eaurora.org>
To: paulmck@...nel.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 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, ...)
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);
>
--
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