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

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Date:	Thu, 8 Jan 2009 09:52:24 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Steven Rostedt <rostedt@...dmis.org>
cc:	Chris Mason <chris.mason@...cle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	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 Thu, 8 Jan 2009, Steven Rostedt wrote:
> 
> We keep spinning if the owner changes.

I think we want to - if you have multiple CPU's and a heavily contended 
lock that acts as a spinlock, we still _do_ want to keep spinning even if 
another CPU gets the lock.

And I don't even believe that is the bug. I suspect the bug is simpler. 

I think the "need_resched()" needs to go in the outer loop, or at least 
happen in the "!owner" case. Because at least with preemption, what can 
happen otherwise is

 - process A gets the lock, but gets preempted before it sets lock->owner.

   End result: count = 0, owner = NULL.

 - processes B/C goes into the spin loop, filling up all CPU's (assuming 
   dual-core here), and will now both loop forever if they hold the kernel 
   lock (or have some other preemption disabling thing over their down()).

And all the while, process A would _happily_ set ->owner, and eventually 
release the mutex, but it never gets to run to do either of them so.

In fact, you might not even need a process C: all you need is for B to be 
on the same runqueue as A, and having enough load on the other CPU's that 
A never gets migrated away. So "C" might be in user space.

I dunno. There are probably variations on the above.

		Linus
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