linux-kernel - Re: [PATCH 1/5] percpu_ida: Fix data race on cpus_have

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Message-ID: <20131028212054.GF8153@kmo>
Date:	Mon, 28 Oct 2013 14:20:54 -0700
From:	Kent Overstreet <kmo@...erainc.com>
To:	Alexander Gordeev <agordeev@...hat.com>
Cc:	Oleg Nesterov <oleg@...hat.com>, Jens Axboe <axboe@...nel.dk>,
	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/5] percpu_ida: Fix data race on cpus_have_tags cpumask

On Mon, Oct 28, 2013 at 11:05:06AM +0100, Alexander Gordeev wrote:
> Function steal_tags() might miss a bit in cpus_have_tags due to
> unsynchronized access from percpu_ida_free(). As result, function
> percpu_ida_alloc() might enter unwakable sleep. This update adds
> memory barriers to prevent the described scenario.
> 
> In fact, accesses to cpus_have_tags are fenced by prepare_to_wait()
> and wake_up() calls at the moment and the aforementioned sequence
> does not appear could hit. Nevertheless, explicit memory barriers
> still seem justifiable.

Good catch!

Acked-by: Kent Overstreet <kmo@...erainc.com>
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