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

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Date:	Tue, 6 Jan 2009 16:06:07 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Matthew Wilcox <matthew@....cx>
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, 6 Jan 2009, Matthew Wilcox wrote:
> > 
> > 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().

No, that's not it.

Look at the callers. They are _all_ the slow path. They looked like this:

	might_sleep();
	return __mutex_lock_common(lock, TASK_KILLABLE, subclass, _RET_IP_);

Yes, you _also_ need to set the owner in the fast-path, but that's all 
entirely different. This is the debug case, which _always_ calls the 
slow-path.

So what I'm saying is that the slow-path should just set it. And then yes, 
we _also_ need to set it in the fast-path, but at least we don't need to 
set it in all the debug versions that just call the slow path!

			Linus
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