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
compilerdoesn'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
shareddata*    structure being accessed within the spinlocked critical section.*    That fixes it because compilers are
notallowed to re-order accesses*    to volatile objects relative to other such accesses.**    On platforms with weak
memoryordering, the TAS(), TAS_SPIN(), and*    S_UNLOCK() macros must further include hardware-level memory fence*
instructionsto prevent similar re-ordering at the hardware level.*    TAS() and TAS_SPIN() must guarantee that loads
andstores issued after*    the macro are not executed until the lock has been obtained.  Conversely,*    S_UNLOCK()
mustguarantee that loads and stores issued before the macro*    have been executed before the lock is released.
 
        regards, tom lane