On Mon, 2015-09-28 at 08:58 +0000, David Laight wrote: > From: Rafael J. Wysocki > > Sent: 27 September 2015 15:09 > ... > > > > Say you have three adjacent fields in a structure, x, y, z, each one byte long. > > > > Initially, all of them are equal to 0. > > > > > > > > CPU A writes 1 to x and CPU B writes 2 to y at the same time. > > > > > > > > What's the result? > > > > > > I think every CPU's cache architecure guarantees adjacent store > > > integrity, even in the face of SMP, so it's x==1 and y==2. If you're > > > thinking of old alpha SMP system where the lowest store width is 32 bits > > > and thus you have to do RMW to update a byte, this was usually fixed by > > > padding (assuming the structure is not packed). However, it was such a > > > problem that even the later alpha chips had byte extensions. > > Does linux still support those old Alphas? > > The x86 cpus will also do 32bit wide rmw cycles for the 'bit' operations. That's different: it's an atomic RMW operation. The problem with the alpha was that the operation wasn't atomic (meaning that it can't be interrupted and no intermediate output states are visible). > > OK, thanks! > > You still have to ensure the compiler doesn't do wider rmw cycles. > I believe the recent versions of gcc won't do wider accesses for volatile data. I don't understand this comment. You seem to be implying gcc would do a 64 bit RMW for a 32 bit store ... that would be daft when a single instruction exists to perform the operation on all architectures. James -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
