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

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Date:	Thu, 08 Jan 2009 13:27:14 -0500
From:	Chris Mason <chris.mason@...cle.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	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, 2009-01-08 at 10:16 -0800, Linus Torvalds wrote:
> 
> On Thu, 8 Jan 2009, Steven Rostedt wrote:
> > 
> > Ouch! I think you are on to something:
> 
> Yeah, there's somethign there, but looking at Chris' backtrace, there's 
> nothing there to disable preemption. So if it was this simple case, it 
> should still have preempted him to let the other process run and finish 
> up.
> 

My .config has no lockdep or schedule debugging and voluntary preempt.
I do have CONFIG_INLINE_OPTIMIZE on, its a good name for trusting gcc I
guess.

> So I don't think Chris' softlockup is at least _exactly_ that case. 
> There's something else going on too.
> 
> That said, I do think it's a mistake for us to care about the value of 
> "spin_on_owner()". I suspect v8 should
> 
>  - always have
> 
> 	if (need_resched())
> 		break
> 
>    in the outer loop.
> 
>  - drop the return value from "spin_on_owner()", and just break out if 
>    anything changes (including the need_resched() flag).
> 
>  - I'd also drop the "old_value < 0 &&" test, and just test the 
>    list_empty() unconditionally. 
> 

I'll give the above a shot, and we can address the preempt + !owner in
another rev

-chris

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