Messages in this thread | | | Date | Tue, 24 Jul 2007 10:24:10 -0700 (PDT) | From | Linus Torvalds <> | Subject | Re: [PATCH 6/8] i386: bitops: Don't mark memory as clobbered unnecessarily |
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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.
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