linux-kernel - Re: [PATCH 2/3] relay: Fix race condition which occurs when reading across CPUs.

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Message-ID: <84144f020806140916j7c39d3fr6ada38169ebc84c2@mail.gmail.com>
Date:	Sat, 14 Jun 2008 19:16:11 +0300
From:	"Pekka Enberg" <penberg@...helsinki.fi>
To:	"Eduard - Gabriel Munteanu" <eduard.munteanu@...ux360.ro>
Cc:	"Tom Zanussi" <tzanussi@...il.com>, akpm@...ux-foundation.org,
	compudj@...stal.dyndns.org, linux-kernel@...r.kernel.org,
	righi.andrea@...il.com
Subject: Re: [PATCH 2/3] relay: Fix race condition which occurs when reading across CPUs.

On Fri, 13 Jun 2008 23:26:41 -0500
Tom Zanussi <tzanussi@...il.com> wrote:
>> Alternatively, you could get rid of the problem by making sure CPU0
>> never reads CPU1's data, by having the userspace reader use per-cpu
>> threads and using sched_setaffinity() to pin each thread to a given
>> cpu. See for example, the blktrace code, which does this.

On Sat, Jun 14, 2008 at 6:11 PM, Eduard - Gabriel Munteanu
<eduard.munteanu@...ux360.ro> wrote:
> Yes, and performance-wise this is better. Though I'm not sure setting
> affinity is 100% safe. Will the thread be migrated soon enough, so we
> don't read cross-CPU? The point is I'm not sure how hard this is
> enforced.
>
> However, I suggest this patch should go in, for two reasons:
> 1. It provides expected behavior in any such situation.
> 2. It adds (almost) no overhead when used in conjuction with setting CPU
> affinity. When the writer acquires the spinlock, it does not busy-wait,
> so the spinlock just disables IRQs (relay_write()).

Agreed. Tom, any objections to merging this patch?
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