Re: [PATCH] sched/completion: completion_done() should serialize with complete()

[Oleg Nesterov <oleg@redhat.com>]

On 02/16, Peter Zijlstra wrote:
>
> On Thu, Feb 12, 2015 at 08:59:13PM +0100, Oleg Nesterov wrote:
> > Commit de30ec47302c "Remove unnecessary ->wait.lock serialization when
> > reading completion state" was not correct, without lock/unlock the code
> > like stop_machine_from_inactive_cpu()
> >
> > 	while (!completion_done())
> > 		cpu_relax();
> >
> > can return before complete() finishes its spin_unlock() which writes to
> > this memory. And spin_unlock_wait().
> >
> > While at it, change try_wait_for_completion() to use READ_ONCE().
>
> So I share Davidlohrs concern

Ah. I forgot to reply to Davidlohr's email. Sorry.

> if we should not simply revert that
> change; but given we've now gone over it detail I suppose we should just
> keep the optimized version.

Yes, I was going to say that of course I won't argue if we simply revert
that commit. As he rigthly pointed the lockless check doesn't make sense
performance-wise.

However, this code needs a comment to explain why we can't simply check
->done and return, unlock_wait() is more documentation than optimization.

But,

> I did add a comment to your patch; and queued the below for
> sched/urgent.

Thanks!

Now this logic is actually documented ;) unlock_wait() alone could confuse
the reader too.

Oleg.