linux-kernel - Re: Serious problem with ticket spinlocks on ia64

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Message-Id: <201008271609.16099.ptesarik@suse.cz>
Date:	Fri, 27 Aug 2010 16:09:14 +0200
From:	Petr Tesarik <ptesarik@...e.cz>
To:	Hedi Berriche <hedi@....com>
Cc:	linux-ia64@...r.kernel.org, linux-kernel@...r.kernel.org,
	Tony Luck <tony.luck@...el.com>
Subject: Re: Serious problem with ticket spinlocks on ia64

On Friday 27 of August 2010 15:48:02 Hedi Berriche wrote:
> On Fri, Aug 27, 2010 at 14:38 Petr Tesarik wrote:
> | Hi everybody,
> |
> | SGI has recently experienced failures with the new ticket spinlock
> | implementation. Hedi Berriche sent me a simple test case that can
> | trigger the failure on the siglock.
>
> One more fact, the problem was introduced by commit
>
>     commit 9d40ee200a527ce08ab8c793ba8ae3e242edbb0e
>     Author: Tony Luck <tony.luck@...el.com>
>     Date:   Wed Oct 7 10:54:19 2009 -0700
>
>         [IA64] Squeeze ticket locks back into 4 bytes.
>
> Reverting the patch makes the problem go away.
>
> IOW, and as far as testing shows, the first incarnation of the ticket locks
> implementation on IA64 (commit 2c8696), the one that used 8 bytes, does not
> exhibit this problem.

I wouldn't be so sure about it. Given that I have only observed the problem 
when the spinlock value wraps around, then an 8-byte spinlock might only need 
much more time to trigger the bug.

Just my two cents,
Petr Tesarik
L3 International
Novell, Inc.
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