[ I found this in the lkml archvive ] > On Wed, Oct 30, 2013 at 04:52:05PM +0200, Victor Kaplansky wrote: > > > Peter Zijlstra <pet...@infradead.org> wrote on 10/30/2013 01:25:26 PM: > > > > > Also, I'm not entirely sure on C, that too seems like a dependency, we > > > simply cannot read the buffer @tail before we've read the tail itself, > > > now can we? Similarly we cannot compare tail to head without having the > > > head read completed. > > > > No, this one we cannot omit, because our problem on consumer side is not > > with @tail, which is written exclusively by consumer, but with @head. > > Ah indeed, my argument was flawed in that @head is the important part. > But we still do a comparison of @tail against @head before we do further > reads. > > Although I suppose speculative reads are allowed -- they don't have the > destructive behaviour speculative writes have -- and thus we could in > fact get reorder issues. > > But since it is still a dependent load in that we do that @tail vs @head > comparison before doing other loads, wouldn't a read_barrier_depends() > be sufficient? Or do we still need a complete rmb? > > > BTW, it is why you also don't need ACCESS_ONCE() around @tail, but only > > around > > @head read. > > Agreed, the ACCESS_ONCE() around tail is superfluous since we're the one > updating tail, so there's no problem with the value changing > unexpectedly.
You need ACCESS_ONCE even if you are the only process writing the value. Because without ACCESS_ONCE, the compiler may perform store tearing and split the store into several smaller stores. Search the file "Documentation/memory-barriers.txt" for the term "store tearing", it shows an example where one instruction storing 32-bit value may be split to two instructions, each storing 16-bit value. Mikulas -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
