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SubjectRe: spin_lock forgets to clobber memory and other smp fixes [was Re: [patch] waitqueue optimization, 2.4.0-test7]
On Mon, 4 Sep 2000, Andrea Arcangeli wrote:

>barrier()). I also noticed __sti()/__save_flags() doesn't need to clobber
>"memory".

I'm not sure anymore if __sti and spin_unlock() doesn't need to clobber
memory (it looks necessary to make sure the compiler doesn't delay to
write data to the memory out of the critical section).

And in practice I can't reproduce any corruption with any subtle testcase
by removing "memory" from the clobber list of all the
spinlocks/__sti/__cli, so it may be the other way around. Maybe we can
rely on the __volatile__ statement of the asm that will enforce that if we
write:

*p = 0;
__asm__ __volatile__("" : :);
*p = 1;

in the assembler we'll then find both a write of 0 and then a write of 1
to memory. I'd say with the current compiler we can rely on it (infact the
spinlock doesn't seems to get miscompiled at the moment even while they
aren't clobbering "memory".

Same with:

int a = *p;
__asm__ __volatile__("" : :);
a = *p;

(to do two explicit reads)

If "memory" isn't necessary in spin_lock, then all the "memory" clobbers
around include/*/* (also in mb() and __cli() and even barrier()) are not
necessary. But then "why" we need a "memory" clobber in first place if the
"memory" isn't necessary? :)) Also the gcc documentation seems to say we
need "memory" in all such operations (also in __sti() and
spin_unlock() unlike I said in my earlier email).
I'd like if some compiler guy could comment (I got no one reply yet). I
tried to grep gcc but my gcc knowledge is too low to reverse engeneer the
implement semantics of the "memory" clobber fast (though I would
appreciate a pointer in the gcc sources to start to understand some gcc
code :).

Andrea

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