linux-kernel - Re: [PATCH] mutex: set owner only once on acquisition

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Message-ID: <20090115085625.GA15038@cmpxchg.org>
Date:	Thu, 15 Jan 2009 09:56:25 +0100
From:	Johannes Weiner <hannes@...xchg.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Gregory Haskins <ghaskins@...ell.com>,
	Matthew Wilcox <matthew@....cx>,
	Andi Kleen <andi@...stfloor.org>,
	Chris Mason <chris.mason@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-btrfs <linux-btrfs@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Nick Piggin <npiggin@...e.de>,
	Peter Morreale <pmorreale@...ell.com>,
	Sven Dietrich <SDietrich@...ell.com>,
	Dmitry Adamushko <dmitry.adamushko@...il.com>
Subject: Re: [PATCH] mutex: set owner only once on acquisition

On Thu, Jan 15, 2009 at 09:41:01AM +0100, Johannes Weiner wrote:
> mutex_lock() sets the lock owner, no need to set it upfront in
> __mutex_lock_common().
> 
> Inside __mutex_lock_common() we can cope with the case where the
> successful acquirer got preempted by us before setting the owner
> field: there is an explicit check in the spinning code and the
> sleeping part doesn't rely on it.
> 
> The debug code does owner checks only on unlock where the field is
> garuanteed to be set.
> 
> Signed-off-by: Johannes Weiner <hannes@...xchg.org>
> ---
>  kernel/mutex.c |    2 --
>  1 file changed, 2 deletions(-)
> 
> Just a small observation.  Peter said it wouldn't matter much as the
> write is to a hot cache line.  But otoh, why keep it if it's not
> necessary. :)

Damn, I'm really async here, sorry Peter.  Just noticed you already
picked it up.

	Hannes
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