linux-kernel - Re: [RFC][PATCH 1/3] locking/mutex: Rework mutex::owner

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Message-ID: <20160824165431.GH16944@arm.com>
Date:   Wed, 24 Aug 2016 17:54:32 +0100
From:   Will Deacon <will.deacon@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Waiman Long <waiman.long@....com>,
        Jason Low <jason.low2@....com>,
        Ding Tianhong <dingtianhong@...wei.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Imre Deak <imre.deak@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Terry Rudd <terry.rudd@....com>,
        "Paul E. McKenney" <paulmck@...ibm.com>,
        Jason Low <jason.low2@...com>
Subject: Re: [RFC][PATCH 1/3] locking/mutex: Rework mutex::owner

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