Messages in this thread | | | Date | Tue, 8 Jul 2014 09:05:05 -0400 (EDT) | From | Mikulas Patocka <> | Subject | Re: [dm-devel] [PATCH] blk-lib: fix error reporting |
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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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