Date: Wed, 7 Jan 2009 18:46:28 -0500 (EST)
From: Steven Rostedt <rostedt@goodmis.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
cc: Gregory Haskins <ghaskins@novell.com>, Ingo Molnar <mingo@elte.hu>,
       Andi Kleen <andi@firstfloor.org>, Matthew Wilcox <matthew@wil.cx>,
       Peter Zijlstra <peterz@infradead.org>, paulmck@linux.vnet.ibm.com,
       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:
> On Wed, 7 Jan 2009, Steven Rostedt wrote:
> > 
> > What would be interesting is various benchmarks against all three.
> > 
> > 1) no mutex spinning.
> > 2) get_task_struct() implementation.
> > 3) spin_or_sched implementation.
> 
> One of the issues is that I cannot convince myself that (2) is even 
> necessarily correct. At least not without having all cases happen inder 
> the mutex spinlock - which they don't. Even with the original patch, the 
> uncontended cases set and cleared the owner field outside the lock.

True. I need to keep looking at the code that is posted. In -rt, we force 
the fast path into the slowpath as soon as another task fails to get the 
lock. Without that, as you pointed out, the code can be racy.

-- Steve

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