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

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Date:	Tue, 08 Jul 2014 07:04:06 -0700
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	device-mapper development <dm-devel@...hat.com>
Cc:	Christoph Hellwig <hch@...radead.org>,
	Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org
Subject: Re: [dm-devel] [PATCH] blk-lib: fix error reporting

On Tue, 2014-07-08 at 09:05 -0400, Mikulas Patocka wrote:
> 
> 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.

That's not correct, because it's not applicable in this case.  Tearing
may occur on misalignment (which ACCESS_ONCE() cannot rectify because
it's architectural), short constant loads (again, usually architectural)
and structure copies, none of which applies here.

We can rely on a properly aligned 32 bit write being atomic.

James


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