Robert Haas <robertmh...@gmail.com> writes: > This discussion seems to miss the fact that there are two levels of > reordering that can happen. First, the compiler can move things > around. Second, the CPU can move things around.
Right, I think that's exactly the problem with the previous wording of that comment; it doesn't address the two logical levels involved. I've rewritten it, see what you think. * Another caution for users of these macros is that it is the caller's * responsibility to ensure that the compiler doesn't re-order accesses * to shared memory to precede the actual lock acquisition, or follow the * lock release. Typically we handle this by using volatile-qualified * pointers to refer to both the spinlock itself and the shared data * structure being accessed within the spinlocked critical section. * That fixes it because compilers are not allowed to re-order accesses * to volatile objects relative to other such accesses. * * On platforms with weak memory ordering, the TAS(), TAS_SPIN(), and * S_UNLOCK() macros must further include hardware-level memory fence * instructions to prevent similar re-ordering at the hardware level. * TAS() and TAS_SPIN() must guarantee that loads and stores issued after * the macro are not executed until the lock has been obtained. Conversely, * S_UNLOCK() must guarantee that loads and stores issued before the macro * have been executed before the lock is released. regards, tom lane -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers