linux-kernel - Re: [PATCH] seqlock: serialize against writers

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Date:	Fri, 29 Aug 2008 12:10:08 -0400
From:	Gregory Haskins <gregory.haskins@...il.com>
To:	Andi Kleen <andi@...stfloor.org>
CC:	Gregory Haskins <ghaskins@...ell.com>, mingo@...e.hu,
	rostedt@...dmis.org, tglx@...utronix.de,
	linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org
Subject: Re: [PATCH] seqlock: serialize against writers

Andi Kleen wrote:
> Gregory Haskins <ghaskins@...ell.com> writes:
>
>   
>> *Patch submitted for inclusion in PREEMPT_RT 26-rt4.  Applies to 2.6.26.3-rt3*
>>
>> Hi Ingo, Steven, Thomas,
>>   Please consider for -rt4.  This fixes a nasty deadlock on my systems under
>>   heavy load.
>>     
>
> Does this even work under x86-64? x86-64 uses seqlocks in user space
> in its vsyscalls. And read_lock() definitely doesn't work there because 
> it writes.
>
> You would need at least to disable vsyscall gettimeofday(), making
> it much much slower.
>
> Perhaps you tested on one of the systems where the vsyscalls need
> to fallback for other reasons? (e.g. one using pmtimer for timing).
>
> -Andi
>   

Im running it on a x86_64 box as we speak.  How can I tell if there is a
certain mode that is permitting this?

-Greg


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