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

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Date:	Tue, 6 Jan 2009 15:00:47 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	paulmck@...ux.vnet.ibm.com, Gregory Haskins <ghaskins@...ell.com>,
	Ingo Molnar <mingo@...e.hu>, Matthew Wilcox <matthew@....cx>,
	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, Peter Zijlstra wrote:
> 
> Indeed, the below does boot -- which means I get to sleep now ;-)

Well, if you didn't go to sleep, a few more questions..

>  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.

IOW, why didn't this just get done with something like

	--- a/kernel/mutex.c
	+++ b/kernel/mutex.c
	@@ -186,6 +186,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
	 done:
	 	lock_acquired(&lock->dep_map, ip);
	 	/* got the lock - rejoice! */
	+	lock->owner = task;
	 	mutex_remove_waiter(lock, &waiter, task_thread_info(task));
	 	debug_mutex_set_owner(lock, task_thread_info(task));

instead?  That takes care of all callers, including the conditional thing 
(since the error case is a totally different path).

		Linus
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