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Message-ID: <alpine.LRH.2.02.1407080859370.27276@file01.intranet.prod.int.rdu2.redhat.com>
Date: Tue, 8 Jul 2014 09:05:05 -0400 (EDT)
From: Mikulas Patocka <mpatocka@...hat.com>
To: Christoph Hellwig <hch@...radead.org>
cc: Jens Axboe <axboe@...nel.dk>, dm-devel@...hat.com,
linux-kernel@...r.kernel.org
Subject: Re: [dm-devel] [PATCH] blk-lib: fix error reporting
On Tue, 8 Jul 2014, Christoph Hellwig wrote:
> > + if (unlikely(err))
> > + ACCESS_ONCE(bb->error) = err;
>
> I can't see a reason for the ACCESS_ONCE here.
Multiple bios can be completed concurrently, so they write bb->error at
the same time. The compiler may do store tearing (see "store tearing" in
Documentation/memory-barriers.txt) - it may split one 4-byte write into
several smaller writes - and it could result in setting bb->error to
invalid value. We need ACCESS_ONCE to make sure that store tearing doesn't
happen.
Mikulas
> Also the likely/unlikely annotations here smell like premature
> optimization.
>
> Otherwise looks good to me.
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