linux-kernel - Re: [PATCH -v7][RFC]: mutex: implement adaptive spinning

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Date:	Fri, 9 Jan 2009 10:24:16 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	Steven Rostedt <rostedt@...dmis.org>,
	Chris Mason <chris.mason@...cle.com>,
	Ingo Molnar <mingo@...e.hu>, paulmck@...ux.vnet.ibm.com,
	Gregory Haskins <ghaskins@...ell.com>,
	Matthew Wilcox <matthew@....cx>,
	Andi Kleen <andi@...stfloor.org>,
	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>
Subject: Re: [PATCH -v7][RFC]: mutex: implement adaptive spinning

On Fri, 9 Jan 2009, Peter Zijlstra wrote:
> 
> On that note:
> 
> Index: linux-2.6/kernel/mutex.c
> ===================================================================
> --- linux-2.6.orig/kernel/mutex.c
> +++ linux-2.6/kernel/mutex.c
> @@ -220,7 +220,9 @@ __mutex_lock_common(struct mutex *lock, 
>  		__set_task_state(task, state);
>  
>  		/* didnt get the lock, go to sleep: */
> +		preempt_disable();
>  		spin_unlock_mutex(&lock->wait_lock, flags);
> +		preempt_enable_no_resched();
>  		schedule();

Yes. I think this is a generic issue independently of the whole adaptive 
thing.

In fact, I think wee could make the mutex code use explicit preemption and 
then the __raw spinlocks to make this more obvious. Because now there's a 
hidden "preempt_enable()" in that spin_unlock_mutex, and anybody looking 
at the code and not realizing it is going to just say "Whaa? Who is this 
crazy Peter Zijlstra guy, and what drugs is he on? I want me some!".

Because your patch really doesn't make much sense unless you know how 
spinlocks work, and if you _do_ know how spinlocks work, you go "eww, 
that's doing extra preemption crud in order to just disable the 
_automatic_ preemption crud".

			Linus
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