Re: spinlocks on HP-UX

Robert Haas <robertmhaas@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