Messages in this thread | | | Subject | Re: [dm-devel] [PATCH] blk-lib: fix error reporting | From | James Bottomley <> | Date | Tue, 08 Jul 2014 07:04:06 -0700 |
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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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