Date: Wed, 7 Jan 2009 15:18:34 -0800 (PST)
From: Linus Torvalds <torvalds@linux-foundation.org>
To: Gregory Haskins <ghaskins@novell.com>
cc: Andi Kleen <andi@firstfloor.org>, Matthew Wilcox <matthew@wil.cx>,
       Steven Rostedt <rostedt@goodmis.org>,
       Peter Zijlstra <peterz@infradead.org>, paulmck@linux.vnet.ibm.com,
       Ingo Molnar <mingo@elte.hu>, Chris Mason <chris.mason@oracle.com>,
       Andrew Morton <akpm@linux-foundation.org>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
       linux-fsdevel <linux-fsdevel@vger.kernel.org>,
       linux-btrfs <linux-btrfs@vger.kernel.org>,
       Thomas Gleixner <tglx@linutronix.de>, Nick Piggin <npiggin@suse.de>,
       Peter Morreale <pmorreale@novell.com>,
       Sven Dietrich <SDietrich@novell.com>
Subject: Re: [PATCH -v5][RFC]: mutex: implement adaptive spinning
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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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