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 -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
