Re: [PATCH 1/1] [RFC] workqueue: fix ghost PENDING flag while doing MQ IO

[Roman Penyaev]

On Mon, Apr 25, 2016 at 7:03 PM, Tejun Heo <tj@xxxxxxxxxx> wrote:
> Hello, Roman.
>
> On Mon, Apr 25, 2016 at 06:34:45PM +0200, Roman Penyaev wrote:
>> I can assure you that smp_mb() helps (at least running for 30 minutes
>> under IO). That was my first variant, but I did not like it because I
>> could not explain myself why:
>>
>> 1. not smp_wmb()? We need to do flush after an update.
>>    (I tried that also, and it does not help)
>
> Regardless of the success of queue_work(), the interface guarantees
> that there will be at least one execution instance which sees whatever
> updates the queuer has made prior to calling queue_work().  The
> PENDING bit is what synchronizes this operations.
>
>         A                               B
>
>                                         Make updates
>         clear PENDING                   test_and_set PENDING
>         start execution
>
> So, if B's test_and_set takes place before clearing of PENDING, what
> should be guaranteed is that A's execution must be able to see B's
> updates; however, as there's no barrier between "clear PENDING" and
> "start execution", memory loads of execution can be scheduled before
> clearing of PENDING which leads to a situation where B loses queueing
> but its updates are not seen by the prior instance's execution.  It's
> a classic "either a sees b (clear PENDING) or b sees a (prior
> updates)" interlocking situation.

Ok, that's clear now. Thanks. I was confused also by a spin lock, which
is being released just after clear pending:

   set_work_pool_and_clear_pending(work, pool->id);
   spin_unlock_irq(&pool->lock);
   ...
   worker->current_func(work);

But seems memory operations of execution can leak-in and appear before
pended bit is cleared and spin lock is released.
(according to Documentation/memory-barriers.txt, (6) RELEASE operations)

>> 2. what protects us from this situation?
>>
>>   CPU#0                  CPU#1
>>                          set_work_data()
>>   test_and_set_bit()
>>                          smp_mb()
>
> The above would be completely fine as CPU#1's execution would see all
> the changes CPU#0 has made upto that point.
>
>> And 2. question was crucial to me, because even tiny delay "fixes" the
>> problem, e.g. ndelay also "fixes" the bug:
>>
>>          smp_wmb();
>>          set_work_data(work, (unsigned long)pool_id << WORK_OFFQ_POOL_SHIFT, 0);
>>  +       ndelay(40);
>>   }
>>
>> Why ndelay(40)? Because on this machine smp_mb() takes 40 ns on average.
>
> Yeah, this is the CPU rescheduling loads for the execution ahead of
> clearing of PENDING and doing anything inbetween is likely to reduce
> the chance of it happening drastically, but smp_mb() inbetween is
> actually the right solution here.

Tejun, do you need an updated patch for that? With a proper smp_mb()?

--
Roman