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Date: Wed, 24 Aug 2016 17:54:32 +0100
From: Will Deacon <will.deacon@arm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Waiman Long <waiman.long@hpe.com>, Jason Low <jason.low2@hpe.com>,
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        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
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        "Paul E. McKenney" <paulmck@us.ibm.com>, Jason Low <jason.low2@hp.com>
Subject: Re: [RFC][PATCH 1/3] locking/mutex: Rework mutex::owner
Message-ID: <20160824165431.GH16944@arm.com>
References: <20160823124617.015645861@infradead.org>
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 <20160824095659.GC16944@arm.com>
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On Wed, Aug 24, 2016 at 06:52:44PM +0200, Peter Zijlstra wrote:
> On Wed, Aug 24, 2016 at 05:34:12PM +0200, Peter Zijlstra wrote:
> > On Wed, Aug 24, 2016 at 10:56:59AM +0100, Will Deacon wrote:
> > > > +	owner = atomic_long_read(&lock->owner);
> > > > +	for (;;) {
> > > > +		unsigned long old;
> > > > +
> > > > +		old = atomic_long_cmpxchg_release(&lock->owner, owner, owner & 0x03);
> > > > +		if (old == owner)
> > > > +			break;
> > > > +
> > > > +		owner = old;
> > > > +	}
> > > 
> > > Can you rewrite this using atomic_long_fetch_and_release?
> > 
> > Yes, until patch 3/3.. but now that you mention it I think we can do:
> > 
> > 	owner = atomic_long_read(&lock->owner);
> > 	if (!(owner & MUTEX_FLAG_HANDOFF))
> > 		(void)atomic_long_fetch_and_release(MUTEX_FLAGS, &lock->owner);
> > 
> 
> And of course, x86 would very much like atomic_long_and_release() here,
> such that it can do LOCK ADD instead of a LOCK CMPXCHG loop. But of
> course, we don't have that ...

... yeah, I noticed that. There is a curious use of atomic_and in
linux/atomic.h, but it's packed full of false promises.

Will