http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55561
--- Comment #31 from Dmitry Vyukov <dvyukov at google dot com> 2013-01-02 10:28:00 UTC --- (In reply to comment #30) > The formatting in the patch is wrong (multiple issues). > > I don't see a point in the __atomic_load_n (addr, MEMMODEL_RELAXED), for > aligned ints or pointers the loads are atomic on all architectures libgomp is > supported on, after all kernel is also using just a normal load in the futex > syscall, not __atomic_load_n (which expands to the normal load only anyway). Do you agree about MEMMODEL_ACQUIRE? Regarding MEMMODEL_RELAXED, there are 2 reasons to use it correctness aside. First, it greatly contributes to code readability and self-documentation, and allows readers to easily distinguish between plain loads and inter-thread synchronization shared loads which are hideously different things. Seconds, it allows tools like tsan to work properly on such code and point to more serious issues (like that MEMMODEL_ACQUIRE above). As for correctness, below is an example that I usually provide (and there is also "How to miscompile programs with benign data races" paper by Boehm with other good examples): ----- Consider that you have an "atomic" (which is not actually marked as atomic for compiler) store and some code w/o sync operations around it (potentially from inlined functions): ... *p = x; ... C++ compiler assumes absence of data races. So if it sees a store to p, then it is allowed to use it as a scratch storage for any garbage in the same synchronization region. I.e. it can do: ... *p = foo; ... *p = x; ... Note that it can't affect any correct race-free program. This way, other threads will read random garbage from p. Now imagine that foo is a function pointer: ... *p = foo; // spill from register ... foo = *p; // restore to register if (bar) foo(); // and execute ... *p = x; ... Now imagine that this thread spills &ReadFile to p, and another thread spills &LaunchNuclearMissle to p in between (but was not intended to execute it due to bar==0). Ooops, this "benign" race just caused accidental launch of nuclear missiles.
