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

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Message-ID: <alpine.LFD.2.00.0901071516280.3057@localhost.localdomain>
Date:	Wed, 7 Jan 2009 15:18:34 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Gregory Haskins <ghaskins@...ell.com>
cc:	Andi Kleen <andi@...stfloor.org>, Matthew Wilcox <matthew@....cx>,
	Steven Rostedt <rostedt@...dmis.org>,
	Peter Zijlstra <peterz@...radead.org>,
	paulmck@...ux.vnet.ibm.com, Ingo Molnar <mingo@...e.hu>,
	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, 7 Jan 2009, Linus Torvalds wrote:
> > 
> > "Is get_task_struct() really that bad?"
> 
> Yes. It's an atomic access (two, in fact, since you need to release it 
> too), which is a huge deal if we're talking about a timing-critical 
> section of code.

There's another issue: you also need to lock the thing that gives you the 
task pointer in the first place. So it's not sufficient to do 
get_task_struct(), you also need to do it within a context where you know 
that the pointer is not going away _while_ you do it.

And with the mutexes clearing the ->owner field without even holding the 
spinlock, that is not a guarantee we can easily get any way. Maybe we'd 
need to hold the tasklist_lock or something.

		Linus
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