From mboxrd@z Thu Jan  1 00:00:00 1970
From: Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH][RFC]: mutex: adaptive spin
Date: Tue, 6 Jan 2009 15:00:47 -0800 (PST)
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Cc: paulmck@linux.vnet.ibm.com, Gregory Haskins <ghaskins@novell.com>,
	Ingo Molnar <mingo@elte.hu>, Matthew Wilcox <matthew@wil.cx>,
	Andi Kleen <andi@firstfloor.org>,
	Chris Mason <chris.mason@oracle.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	linux-kernel@vger.kernel.org,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	linux-btrfs <linux-btrfs@vger.kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Steven Rostedt <rostedt@goodmis.org>,
	Nick Piggin <npiggin@suse.de>,
	Peter Morreale <pmorreale@novell.com>,
	Sven Dietrich <SDietrich@novell.com>
To: Peter Zijlstra <peterz@infradead.org>
Return-path: <linux-fsdevel-owner@vger.kernel.org>
In-Reply-To: <1231281801.11687.125.camel@twins>
List-ID: <linux-btrfs.vger.kernel.org>



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