linux-kernel - Re: [RFC 2/3] mutex: Modify the way optimistic spinners are queued

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Date:	Wed, 15 Jan 2014 11:23:01 -0800
From:	Jason Low <jason.low2@...com>
To:	Waiman Long <waiman.long@...com>
Cc:	mingo@...hat.com, peterz@...radead.org, paulmck@...ux.vnet.ibm.com,
	torvalds@...ux-foundation.org, tglx@...utronix.de,
	linux-kernel@...r.kernel.org, riel@...hat.com,
	akpm@...ux-foundation.org, davidlohr@...com, hpa@...or.com,
	aswin@...com, scott.norton@...com
Subject: Re: [RFC 2/3] mutex: Modify the way optimistic spinners are queued

On Wed, 2014-01-15 at 10:10 -0500, Waiman Long wrote:
> On 01/14/2014 07:33 PM, Jason Low wrote:
> >   		 * When there's no owner, we might have preempted between the
> > @@ -503,8 +504,10 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
> >   		 * we're an RT task that will live-lock because we won't let
> >   		 * the owner complete.
> >   		 */
> > -		if (!owner&&  (need_resched() || rt_task(task)))
> > +		if (!owner&&  (need_resched() || rt_task(task))) {
> > +			mspin_unlock(MLOCK(lock),&node);
> >   			goto slowpath;
> > +		}
> >
> >   		/*
> >   		 * The cpu_relax() call is a compiler barrier which forces
> 
> Maybe you can consider restructure the code as follows to reduce the 
> number of mspin_unlock() call sites:

Yeah, I would prefer your method of using break and having the
mspin_unlock() at the end of the loop, now that it would result in less
# of mspin_unlock().

Commit ec83f425dbca47e19c6737e8e7db0d0924a5de1b changed break to
slowpath to make it more intuitive to read, but with this patch, there
are benefits to using break.

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