On Wednesday, December 05, 2012, Leif Walsh wrote:
> Here are the threads I have:
>
> {
> pthread_rwlock_rdlock();
> x = 1;
> pthread_rwlock_rdunlock();
> }
>
> {
> pthread_rwlock_wrlock();
> if (x) {
> x = 0;
> } else {
> foo();
> }
> pthread_rwlock_wrunlock();
> }
>
> Boehm's paper did not convince me of a bug.
>
> Since pthread locks will include proper mfences I believe this to be
> correct even on non-x86 machines.
Well, but what's your definition of "correct"? That's central to the
issue at hand.
Imagine that pthread_rwlock_rdunlock contains a one-sided fence,
that causes all writes done by other processors to be visible on
this processor, but has no effect on migrating writes done by this
processor to others. (Why should it? Holding a reader lock doesn't
allow you to write shared memory, so a conforming program should
have no writes that need to be propagated to other processors at
lock release time.)
The above is just a guess. I'm not saying your code will fail on current
implementations. Merely that (as Patrick points out), non-adherence
to the standard puts the program pretty much in the Russian Roulette
department w.r.t. future hardware and compiler developments.
You may remember the gcc -fstrict-aliasing wars of the early 2000s,
in which the gcc developers starting using a relatively obscure aspect
of the C/C++ standard to improve optimisation. As a result vast reams
of previously-working code (including Valgrind) started to fail in
obscure ways.
Same thing happened on a smaller scale a couple of years later when
gcc started to assume that signed integer arithmetic would never overflow.
So .. seen from a historical context, the compiler crews are an extremely
ingenious bunch, and I would not be at all surprised to see newer compilers
causing non-compliant (racey) code to fail more often.
J
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