linux-kernel - Re: RFC: mutex: hung tasks on SMP platforms with asm-generic/mutex-xchg.h

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Date:	Thu, 9 Aug 2012 19:17:05 +0100
From:	Will Deacon <will.deacon@....com>
To:	Nicolas Pitre <nico@...xnic.net>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Chris Mason <chris.mason@...ionio.com>,
	Arnd Bergmann <arnd@...db.de>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: RFC: mutex: hung tasks on SMP platforms with
 asm-generic/mutex-xchg.h

On Thu, Aug 09, 2012 at 07:09:02PM +0100, Nicolas Pitre wrote:
> On Thu, 9 Aug 2012, Will Deacon wrote:
> > On Thu, Aug 09, 2012 at 05:57:33PM +0100, Nicolas Pitre wrote:
> > > On Thu, 9 Aug 2012, Nicolas Pitre wrote:
> > > diff --git a/include/asm-generic/mutex-xchg.h b/include/asm-generic/mutex-xchg.h
> > > index 580a6d35c7..44a66c99c8 100644
> > > --- a/include/asm-generic/mutex-xchg.h
> > > +++ b/include/asm-generic/mutex-xchg.h
> > > @@ -25,8 +25,11 @@
> > >  static inline void
> > >  __mutex_fastpath_lock(atomic_t *count, void (*fail_fn)(atomic_t *))
> > >  {
> > > -	if (unlikely(atomic_xchg(count, 0) != 1))
> > > -		fail_fn(count);
> > > +	if (unlikely(atomic_xchg(count, 0) != 1)) {
> > > +		/* Mark lock contention explicitly */
> > > +		if (likely(atomic_xchg(count, -1) != 1))
> > > +			fail_fn(count);
> > > +	}
> > >  }
> > >  
> > >  /**
> > 
> > Doesn't this mean that we're no longer just swapping 0 for a 0 if the lock
> > was taken, therefore needlessly sending the current owner down the slowpath
> > on unlock?
> 
> If the lock was taken, this means the count was either 0 or -1.  If it 
> was 1 then we just put a 0 there and we own it.  But if the cound was 0 
> then we should store -1 instead, which is what the inner xchg does.  If 
> the count was already -1 then we store -1 back.  That more closely mimic 
> what the atomic dec does which is what we want.

Ok, I just wasn't sure that marking the lock contended was required when it
was previously locked, given that we'll drop into spinning on the owner
anyway.

I'll add a commit message to the above and re-post if that's ok?

Will
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