linux-kernel - Re: [PATCH/RFC] mutex: Fix optimistic spinning vs. BKL

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Message-ID: <1272429574.24542.84.camel@pasglop>
Date:	Wed, 28 Apr 2010 14:39:34 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...e.hu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Tony Breeds <tonyb@....ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH/RFC] mutex: Fix optimistic spinning vs. BKL

On Wed, 2010-04-28 at 14:38 +1000, Benjamin Herrenschmidt wrote:
> Currently, we can hit a nasty case with optimistic spinning on mutexes:
> 
>     CPU A tries to take a mutex, while holding the BKL
> 
>     CPU B tried to take the BLK while holding the mutex
> 
> This looks like a AB-BA scenario but in practice, is allowed and happens
> due to the auto-release-on-schedule nature of the BKL.

 .../...

BTW. The patch is only compile-tested so far :-) It's going to be
hammered with the test case that triggered the original bug hopefully
tonight.

Cheers,
Ben.


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