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

[Tejun Heo]

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.

> 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.

Thanks.

-- 
tejun