Date:   Fri, 22 Nov 2019 03:44:41 +0000
From:   Neeraj Upadhyay <neeraju@...eaurora.org>
To:     paulmck@...nel.org
Cc:     josh@...htriplett.org, joel@...lfernandes.org, rostedt@...dmis.org,
        mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
        linux-kernel@...r.kernel.org, rcu@...r.kernel.org,
        pkondeti@...eaurora.org, prsood@...eaurora.org,
        gkohli@...eaurora.org
Subject: Re: [PATCH] rcu: Fix missed wakeup of exp_wq waiters



On 11/22/2019 12:25 AM, Paul E. McKenney wrote:
> On Thu, Nov 21, 2019 at 10:06:40AM +0530, Neeraj Upadhyay wrote:
>>
>>
>> On 11/21/2019 9:40 AM, Paul E. McKenney wrote:
>>> On Wed, Nov 20, 2019 at 10:38:56AM +0530, Neeraj Upadhyay wrote:
>>>>
>>>>
>>>> On 11/20/2019 1:36 AM, Paul E. McKenney wrote:
>>>>> On Tue, Nov 19, 2019 at 07:09:31AM -0800, Paul E. McKenney wrote:
>>>>>> On Tue, Nov 19, 2019 at 07:03:14AM +0000, Neeraj Upadhyay wrote:
>>>>>>> Hi Paul,
>>>>>>>
>>>>>>> On 11/19/2019 9:35 AM, Paul E. McKenney wrote:
>>>>>>>> On Tue, Nov 19, 2019 at 03:35:15AM +0000, Neeraj Upadhyay wrote:
>>>>>>>>> Hi Paul,
>>>>>>>>>
>>>>>>>>> On 11/18/2019 10:54 PM, Paul E. McKenney wrote:
>>>>>>>>>> On Mon, Nov 18, 2019 at 04:41:47PM +0000, Neeraj Upadhyay wrote:
>>>>>>>>>>> Hi Paul,
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> On 11/18/2019 8:38 PM, Paul E. McKenney wrote:
>>>>>>>>>>>> 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.
>>>>>>>>>
>>>>>>>>> Done.
>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>> 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.
>>>>>>>>>
>>>>>>>>> The reason for highlighting the alternate approach of doing gp end inside
>>>>>>>>> exp_wake_mutex is the requirement of 3 wqs. Now, this is a theoretical case;
>>>>>>>>> please correct me if I am wrong here:
>>>>>>>>>
>>>>>>>>> 1. task0 holds exp_wake_mutex, and is preempted.
>>>>>>>>
>>>>>>>> Presumably after it has awakened the kthread that initiated the prior
>>>>>>>> expedited grace period (the one with seq number = -4).
>>>>>>>>
>>>>>>>>> 2. task1 initiates new GP (current seq number = 0).
>>>>>>>>
>>>>>>>> Yes, this can happen.
>>>>>>>>
>>>>>>>>> 3. task1 queues worker kworker1 and schedules out.
>>>>>>>>
>>>>>>>> And thus still holds .exp_mutex, but yes.
>>>>>>>>
>>>>>>>>> 4. kworker1 sets exp GP to 1 and waits on exp_wake_mutex
>>>>>>>>
>>>>>>>> And thus cannot yet have awakened task1.
>>>>>>>>
>>>>>>>>> 5. task1 releases exp mutex, w/o entering waitq.
>>>>>>>>
>>>>>>>> So I do not believe that we can get to #5.  What am I missing here?
>>>>>>>>
>>>>>>>
>>>>>>> As mentioned in this patch, task1 could have scheduled out after queuing
>>>>>>> work:
>>>>>>>
>>>>>>> queue_work(rcu_gp_wq, &rew.rew_work)
>>>>>>>               wake_up_worker()
>>>>>>>                 schedule()
>>>>>>>
>>>>>>> kworker1 runs and picks up this queued work, and sets exp GP to 1 and waits
>>>>>>> on exp_wake_mutex.
>>>>>>>
>>>>>>> task1 gets scheduled in and checks sync_exp_work_done(rsp, s), which return
>>>>>>> true and it does not enter wait queue and releases exp_mutex.
>>>>>>>
>>>>>>> wait_event(rnp->exp_wq[rcu_seq_ctr(s) & 0x3],
>>>>>>>             sync_exp_work_done(rsp, s));
>>>>>>
>>>>>> Well, I have certainly given enough people a hard time about missing the
>>>>>> didn't-actually-sleep case, so good show on finding one in my code!  ;-)
>>>>>>
>>>>>> Which also explains why deferring the rcu_exp_gp_seq_end() is safe:
>>>>>> The .exp_mutex won't be released until after it happens, and the
>>>>>> next manipulation of the sequence number cannot happen until after
>>>>>> .exp_mutex is next acquired.
>>>>>>
>>>>>> Good catch!  And keep up the good work!!!
>>>>>
>>>>> And here is the commit corresponding to your earlier patch.  Please let
>>>>> me know of any needed adjustments.
>>>>>
>>>>> 							Thanx, Paul
>>>>>
>>>>> ------------------------------------------------------------------------
>>>>>
>>>>> commit 3ec440b52831eea172061c5db3d2990b22904863
>>>>> Author: Neeraj Upadhyay <neeraju@...eaurora.org>
>>>>> Date:   Tue Nov 19 11:50:52 2019 -0800
>>>>>
>>>>>        rcu: Allow only one expedited GP to run concurrently with wakeups
>>>>>        The current expedited RCU grace-period code expects that a task
>>>>>        requesting an expedited grace period cannot awaken until that grace
>>>>>        period has reached the wakeup phase.  However, it is possible for a long
>>>>>        preemption to result in the waiting task never sleeping.  For example,
>>>>>        consider the following sequence of events:
>>>>>        1.      Task A starts an expedited grace period by invoking
>>>>>                synchronize_rcu_expedited().  It proceeds normally up to the
>>>>>                wait_event() near the end of that function, and is then preempted
>>>>>                (or interrupted or whatever).
>>>>>        2.      The expedited grace period completes, and a kworker task starts
>>>>>                the awaken phase, having incremented the counter and acquired
>>>>>                the rcu_state structure's .exp_wake_mutex.  This kworker task
>>>>>                is then preempted or interrupted or whatever.
>>>>>        3.      Task A resumes and enters wait_event(), which notes that the
>>>>>                expedited grace period has completed, and thus doesn't sleep.
>>>>>        4.      Task B starts an expedited grace period exactly as did Task A,
>>>>>                complete with the preemption (or whatever delay) just before
>>>>>                the call to wait_event().
>>>>>        5.      The expedited grace period completes, and another kworker
>>>>>                task starts the awaken phase, having incremented the counter.
>>>>>                However, it blocks when attempting to acquire the rcu_state
>>>>>                structure's .exp_wake_mutex because step 2's kworker task has
>>>>>                not yet released it.
>>>>>        6.      Steps 4 and 5 repeat, resulting in overflow of the rcu_node
>>>>>                structure's ->exp_wq[] array.
>>>>>        In theory, this is harmless.  Tasks waiting on the various ->exp_wq[]
>>>>>        array will just be spuriously awakened, but they will just sleep again
>>>>>        on noting that the rcu_state structure's ->expedited_sequence value has
>>>>>        not advanced far enough.
>>>>>        In practice, this wastes CPU time and is an accident waiting to happen.
>>>>>        This commit therefore moves the rcu_exp_gp_seq_end() call that officially
>>>>>        ends the expedited grace period (along with associate tracing) until
>>>>>        after the ->exp_wake_mutex has been acquired.  This prevents Task A from
>>>>>        awakening prematurely, thus preventing more than one expedited grace
>>>>>        period from being in flight during a previous expedited grace period's
>>>>>        wakeup phase.
>>>>
>>>> I am not sure, if a "fixes" tag is required for it.
>>>
>>> If you have a suggested commit, I would be happy to add it.
>>>
>>
>> I think either or below 2 - first one is on the tree_exp.h file, second
>> one looks to be the original commit.
>>
>> 1. https://github.com/torvalds/linux/commit/3549c2bc2c4ea8ecfeb9d21cb81cb00c6002b011
>>
>> 2. https://github.com/torvalds/linux/commit/3b5f668e715bc19610ad967ef97a7e8c55a186ec
> 
> Agreed, this second commit is the one that introduced the bug.  I placed
> "Fixes:" tags on both of your commits for this one.  And thank you for
> digging them both up!
> 
> 							Thanx, Paul
> 

No problem.

>> Thanks
>> Neeraj
>>
>>>>>        Signed-off-by: Neeraj Upadhyay <neeraju@...eaurora.org>
>>>>>        [ paulmck: Added updated comment. ]
>>>>>        Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
>>>>>
>>>>> diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
>>>>> index 4433d00a..8840729 100644
>>>>> --- a/kernel/rcu/tree_exp.h
>>>>> +++ b/kernel/rcu/tree_exp.h
>>>>> @@ -539,14 +539,13 @@ static void rcu_exp_wait_wake(unsigned long s)
>>>>>     	struct rcu_node *rnp;
>>>>>     	synchronize_sched_expedited_wait();
>>>>> -	rcu_exp_gp_seq_end();
>>>>> -	trace_rcu_exp_grace_period(rcu_state.name, s, TPS("end"));
>>>>> -	/*
>>>>> -	 * Switch over to wakeup mode, allowing the next GP, but -only- the
>>>>> -	 * next GP, to proceed.
>>>>> -	 */
>>>>> +	// Switch over to wakeup mode, allowing the next GP to proceed.
>>>>> +	// End the previous grace period only after acquiring the mutex
>>>>> +	// to ensure that only one GP runs concurrently with wakeups.
>>>>
>>>> Should comment style be changed to below?
>>>>
>>>> /* Switch over to wakeup mode, allowing the next GP to proceed.
>>>>    * End the previous grace period only after acquiring the mutex
>>>>    * to ensure that only one GP runs concurrently with wakeups.
>>>>    */
>>>
>>> No.  "//" is acceptable comment format, aside from docbook headers.
>>> The "//" approach saves three characters per line compared to "/* ... */"
>>> single-line comments and a line compared to the style you show above.
>>>
>>> So yes, some maintainers prefer the style you show, but not me.
>>>
>>> 							Thanx, Paul
>>>

Got it.


Thanks
Neeraj

>>>>>     	mutex_lock(&rcu_state.exp_wake_mutex);
>>>>> +	rcu_exp_gp_seq_end();
>>>>> +	trace_rcu_exp_grace_period(rcu_state.name, s, TPS("end"));
>>>>>     	rcu_for_each_node_breadth_first(rnp) {
>>>>>     		if (ULONG_CMP_LT(READ_ONCE(rnp->exp_seq_rq), s)) {
>>>>>
>>>>
>>>> -- 
>>>> 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

-- 
QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a 
member of the Code Aurora Forum, hosted by The Linux Foundation

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