Re: [PATCH 6/8] i386: bitops: Don't mark memory as clobbered unnecessarily

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Tue, 2007-07-24 at 10:24 -0700, Linus Torvalds wrote:
> 
> On Tue, 24 Jul 2007, Benjamin Herrenschmidt wrote:
> > 
> > In fact, it's more than that... the bitops that return a value are often
> > used to have hand-made spinlock semantics. I'm sure we would get funky
> > bugs if loads or stores leaked out of the locked region. I think a full
> > "memory" clobber should be kept around for those cases.
> 
> Not helpful.
> 
> The CPU ordering constraints for "test_and_set_bit()" and friends are weak 
> enough that even if you have a full memory clobber, it still wouldn't work 
> as a lock.
>
> That's exactly why we have smp_mb__after_set_bit() and friends. On some 
> architectures (arm, mips, parsic, powerpc), *that* is where the CPU memory 
> barrier is, because the "test_and_set_bit()" itself is just a 
> cache-coherent operation, not an actual barrier.

Well, as I said, our test_and_set_bit() asm (and in general, the asm for
all the atomic ops that -return- a value) have at least some level of
barriers in them because of that. We do that because people are abusing
them as locks. The smp_mb__after_set_bit() I never quite grokked. We do
an mb in there but I suspect we don't need if it's only ever used after
test_and_set_bit() because of the above. The smb_mb__before_clear_bit()
makes more sense as it's supposed to give clear_bit() a spin_unlock
semantic.

But we do need the "memory" clobber as well.

That's one reason why I like Nick's bitop locks patches, providing
-explicit- test_and_set_bit_lock() and clear_bit_unlock(), we can fix a
whole lot of things and make sure they have the right barriers and not
more. (We save a few useless barriers on POWER that way in the page lock
path and it's measureable in his benchmark).

Cheers,
Ben.