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

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Date:	Wed, 07 Jan 2009 23:00:18 -0500
From:	Gregory Haskins <ghaskins@...ell.com>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>,
	Matthew Wilcox <matthew@....cx>,
	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

Steven Rostedt wrote:
> On Wed, 7 Jan 2009, Gregory Haskins wrote:
>   
>> In my defense, the -rt versions of the patches guarantee this is ok
>> based on a little hack:
>>     
>
> The -rt versions worry about much more than what the mutex code in
> mainline does. Linus is correct in his arguments. The adaptive mutex (as 
> suppose to what -rt has), is only to help aid in preformance. There are a 
> lot of races that can happen in mainline version where lock taking may not 
> be fifo, or where we might start to schedule when we could have taken the 
> lock. These races are not in -rt, but that is because -rt cares about 
> these. But mainline cares more about performance over determinism. This 
> means that we have to look at the current code that Peter is submitting 
> with a different perspective than we do in -rt.
>   

Hey Steve,
  Understood, and agreed.  I only mentioned it because I wanted to clear
the record that I did not (to my knowledge) mess up the protocol design
which first introduced the get/put-task pattern under discussion ;).  I
am fairly confident that at least the -rt version does not have any race
conditions such as the one Linus mentioned in the mainline version.  I
am not advocating that the full protocol that we use in -rt should be
carried forward, per se or anything like that.

-Greg



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