Message-ID: <0101016e81ba8949-6141af5c-f8f7-404d-a8c0-41b270c532c6-000000@us-west-2.amazonses.com>
Date:   Tue, 19 Nov 2019 03:35:16 +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

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.
2. task1 initiates new GP (current seq number = 0).
3. task1 queues worker kworker1 and schedules out.
4. kworker1 sets exp GP to 1 and waits on exp_wake_mutex
5. task1 releases exp mutex, w/o entering waitq.
6. task2 initiates new GP (current seq number = 1).
7. task2 queues worker kworker1 and schedules out.
8. kworker 2 sets exp GP to 2 and waits on exp_wake_mutex.
...

This sequence would break the requirement of max 3 wqs.

If we hold the exp_wake_mutex before exp seq end, there will be atmost 
three pending GPs - 1. GP which current owner is doing wakeups for,
2. GP which the waiter of exp_wake_mutex would complete, 3. Next GP
waiters, which started after GP @ point 2 had started. This also is 
inline with the statement in [1]:

"he key point is that the ->exp_mutex is not released until
the first wakeup is complete, which means that the ->exp_wake_mutex
has already been acquired at that point"

[1] 
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

>>>
>>> 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.)
>>
>> My bad, I posted incomplete diff in previous mail:
>>
>>   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);
>> +		wake_up_all(&rnp->exp_wq[exp_low]);
> 
> Much better!
> 
> But I suggest using s_low in the wake_up_all.  This hunk is of course
> only for testing purposes, not for actual inclusion.  (My earlier email
> didn't make that clear.)
> 
> 							Thanx, Paul
> 

Ok sure, got it. I will share the results, once the issue is reproduced.


Thanks
Neeraj

>>   	}
>>
>>
>> Thanks
>> Neeraj
>>
>>>
>>> 							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@...eaurora.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
>>
>> -- 
>> 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

Powered by blists - more mailing lists