On Thu, 21 Jun 2007, Ingo Molnar wrote:
>
> yeah. I think Linux is i think the only OS on the planet that is using
> the movb trick for unlock, it even triggered a hardware erratum ;)
I'm pretty sure others do it too.
Maybe not on an OS level (but I actually doubt that - I'd be surprised if
Windows doesn't do the exact same thing), but I know for a fact that a lot
of people in threaded libraries end up depending very much on the "simple
store closes a locked section".
Just googling for "xchg" "mov" "spinlock" "-linux" shows discussion boards
for Windows developers with open-coded spinlocks like
int ResourceFlag = 0; // 0=Free, 1=Inuse
...
// Wait until we get the resource
while(InterlockedExchange(&ResourceFlag, 1) != 0) {
Sleep(0); } // Wait a tad
// Have the resource
... // do your thing
ResourceFlag = 0; // Release the resource
and that's definitely Windows code, not some Linux person doing it.
And this is from an OS2 forum
unsigned owned=0;
void request() {
while(LockedExchanged(&owned,1)!=0)
;
}
void release() {
owned = 0;
}
so it's not even something unusual.
So while arguably these people don't know (and don't care) about subtle
issues like memory ordering, I can *guarantee* that a lot of programs
depend on them, even if that dependency may often come from a lack of
knowledge, rather than actively understanding what we do like in the Linux
kernel community.
(And yes, they rely on compilers not reordering either. Tough.)
Linus
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