[dm-devel] [PATCH] blk-lib: fix error reporting
James.Bottomley at HansenPartnership.com
Tue Jul 8 14:04:06 UTC 2014
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
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.
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