On Thu, 21 Jun 2007, Ingo Molnar wrote:
>
> * Linus Torvalds <[email protected]> wrote:
>
> > If somebody can actually come up with a sequence where we have
> > spinlock starvation, and it's not about an example of bad locking, and
> > nobody really can come up with any other way to fix it, we may
> > eventually have to add the notion of "fair spinlocks".
>
> there was one bad case i can remember: the spinlock debugging code had a
> trylock open-coded loop and on certain Opterons CPUs were starving each
> other.
But this is a perfect example of exactly what I'm talking about:
THAT CODE IS HORRIBLY BUGGY!
It's not the spinlocks that are broken, it's that damn code.
> for (;;) {
> for (i = 0; i < loops; i++) {
> if (__raw_write_trylock(&lock->raw_lock))
> return;
> __delay(1);
> }
What a piece of crap.
Anybody who ever waits for a lock by busy-looping over it is BUGGY,
dammit!
The only correct way to wait for a lock is:
(a) try it *once* with an atomic r-m-w
(b) loop over just _reading_ it (and something that implies a memory
barrier, _not_ "__delay()". Use "cpu_relax()" or "smp_rmb()")
(c) rinse and repeat.
and code like the above should just be shot on sight.
So don't blame the spinlocks or the hardware for crap code.
Linus
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