linux-kernel - Re: [PATCH v8 09/14] lockdep: Apply crossrelease to completions

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Date:   Thu, 10 Aug 2017 10:24:31 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...nel.org, tglx@...utronix.de, walken@...gle.com,
        boqun.feng@...il.com, kirill@...temov.name,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org, willy@...radead.org, npiggin@...il.com,
        kernel-team@....com
Subject: Re: [PATCH v8 09/14] lockdep: Apply crossrelease to completions

On Wed, Aug 09, 2017 at 12:24:39PM +0200, Peter Zijlstra wrote:
> On Wed, Aug 09, 2017 at 11:51:07AM +0200, Peter Zijlstra wrote:
> > On Mon, Aug 07, 2017 at 04:12:56PM +0900, Byungchul Park wrote:
> > > +static inline void wait_for_completion(struct completion *x)
> > > +{
> > > +	complete_acquire(x);
> > > +	__wait_for_completion(x);
> > > +	complete_release(x);
> > > +}
> > > +
> > > +static inline void wait_for_completion_io(struct completion *x)
> > > +{
> > > +	complete_acquire(x);
> > > +	__wait_for_completion_io(x);
> > > +	complete_release(x);
> > > +}
> > > +
> > > +static inline int wait_for_completion_interruptible(struct completion *x)
> > > +{
> > > +	int ret;
> > > +	complete_acquire(x);
> > > +	ret = __wait_for_completion_interruptible(x);
> > > +	complete_release(x);
> > > +	return ret;
> > > +}
> > > +
> > > +static inline int wait_for_completion_killable(struct completion *x)
> > > +{
> > > +	int ret;
> > > +	complete_acquire(x);
> > > +	ret = __wait_for_completion_killable(x);
> > > +	complete_release(x);
> > > +	return ret;
> > > +}
> > 
> > I don't understand, why not change __wait_for_common() ?
> 
> That is what is wrong with the below?
> 
> Yes, it adds acquire/release to the timeout variants too, but I don't

Yes, I didn't want to involve them in lockdep play which reports _deadlock_
warning since it's not a dependency causing a deadlock.

> see why we should exclude those, and even if we'd want to do that, it
> would be trivial:
> 
> 	bool timo = (timeout == MAX_SCHEDULE_TIMEOUT);
> 
> 	if (!timo)
> 		complete_acquire(x);
> 
> 	/* ... */
> 
> 	if (!timo)
> 		complete_release(x);

Yes, frankly I wanted to use this.. but skip it.

> But like said, I think we very much want to annotate waits with timeouts
> too. Hitting the max timo doesn't necessarily mean we'll make fwd
> progress, we could be stuck in a loop doing something else again before
> returning to wait.

In that case, it should be detected by other dependencies which makes
problems, not the dependency by wait_for_complete().

> Also, even if we'd make fwd progress, hitting that max timo is still not
> desirable.

It's not desirable but it's not a dependency causing a deadlock, so I did
not want to _deadlock_ warning in that cases.. I didn't want to abuse
lockdep reports..

However, it's OK if you think it's worth warning even in that cases.

Thank you very much,
Byungchul