Re: Intermediate report for AIX 5L port

> Got it.  The AIX compiler apparently feels free to rearrange the
> sequence
> 
>         proc->lwWaiting = true;
>         proc->lwExclusive = (mode == LW_EXCLUSIVE);
>         proc->lwWaitLink = NULL;
>         if (lock->head == NULL)
>             lock->head = proc;
>         else
>             lock->tail->lwWaitLink = proc;
>         lock->tail = proc;
> 
>         /* Can release the mutex now */
>         SpinLockRelease_NoHoldoff(&lock->mutex);
> 
> into something wherein the SpinLockRelease (which is just "x = 0")
> occurs before the last two assignments into the lock structure.
> Boo, hiss.  Evidently, on your multiprocessor machine, there may be
> another CPU that is able to obtain the spinlock and then read the
> un-updated lock values before the stores occur.
> 
> Declaring the lock pointer "volatile" seems to prevent this misbehavior.
> 
> Personally I'd call this a compiler bug; isn't it supposed to consider
> semicolons as sequence points?  I never heard that rearranging the order
> of stores into memory was considered a kosher optimization of C code.

Looks funny to me too. I will let the IBM engineers know what you have
found. Thanks.
--
Tatsuo Ishii