linux-kernel - Re: [PATCH][RFC]: mutex: adaptive spin

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Date:	Tue, 6 Jan 2009 16:09:53 -0700
From:	Matthew Wilcox <matthew@....cx>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Peter Zijlstra <peterz@...radead.org>, paulmck@...ux.vnet.ibm.com,
	Gregory Haskins <ghaskins@...ell.com>,
	Ingo Molnar <mingo@...e.hu>, Andi Kleen <andi@...stfloor.org>,
	Chris Mason <chris.mason@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-btrfs <linux-btrfs@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Nick Piggin <npiggin@...e.de>,
	Peter Morreale <pmorreale@...ell.com>,
	Sven Dietrich <SDietrich@...ell.com>
Subject: Re: [PATCH][RFC]: mutex: adaptive spin

On Tue, Jan 06, 2009 at 03:00:47PM -0800, Linus Torvalds wrote:
> Well, if you didn't go to sleep, a few more questions..

I know this one!  Me sir, me me me!

> >  int __sched
> >  mutex_lock_killable_nested(struct mutex *lock, unsigned int subclass)
> >  {
> > +	int ret;
> > +
> >  	might_sleep();
> > -	return __mutex_lock_common(lock, TASK_KILLABLE, subclass, _RET_IP_);
> > +	ret =  __mutex_lock_common(lock, TASK_KILLABLE, subclass, _RET_IP_);
> > +	if (!ret)
> > +		lock->owner = current;
> > +
> > +	return ret;
> 
> This looks ugly. Why doesn't __mutex_lock_common() just set the lock 
> owner? Hate seeing it done in the caller that has to re-compute common 
> (yeah, yeah, it's cheap) and just looks ugly.

Because __mutex_lock_common() is the slow path.  The fast path is a
couple of assembly instructions in asm/mutex.h.  If the lock isn't
contended, it will never call __mutex_lock_common().

That would make the whole exercise rather pointless; the only time worth
spinning really is if you're the only other one waiting for it ... if
there's already a waiter, you might as well go to sleep.

-- 
Matthew Wilcox				Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."
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