linux-kernel - Re: [PATCH -v5][RFC]: mutex: implement adaptive spinning

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Date:	Wed, 7 Jan 2009 14:09:23 -0700
From:	Matthew Wilcox <matthew@....cx>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	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 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 -v5][RFC]: mutex: implement adaptive spinning

On Wed, Jan 07, 2009 at 12:55:49PM -0800, Linus Torvalds wrote:
> 	void loop_while_oncpu(struct mutex *lock, struct thread_struct *thread)
> 	{
> 		for (;;) {
> 			unsigned cpu;
> 			struct runqueue *rq;
> 
> 			if (lock->owner != thread)
> 				break;
> 
> 			/*
> 			 * Need to access the cpu field knowing that
> 			 * DEBUG_PAGEALLOC could have unmapped it if
> 			 * the mutex owner just released it and exited.
> 			 */
> 			if (__get_user(cpu, &thread->cpu))
> 				break;

I appreciate this is sample code, but using __get_user() on
non-userspace pointers messes up architectures which have separate
user/kernel spaces (eg the old 4G/4G split for x86-32).  Do we have an
appropriate function for kernel space pointers?  Is this a good reason
to add one?

-- 
Matthew Wilcox				Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."
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