linux-kernel - Re: [dm-devel] [PATCH] blk-lib: fix error reporting

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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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