Messages in this thread | | | Subject | Re: [PATCH 6/8] i386: bitops: Don't mark memory as clobbered unnecessarily | From | Trond Myklebust <> | Date | Tue, 24 Jul 2007 13:42:12 -0400 |
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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.
That's not what the Documentation/memory-barriers.txt states:
Any atomic operation that modifies some state in memory and returns information about the state (old or new) implies an SMP-conditional general memory barrier (smp_mb()) on each side of the actual operation. These include:
..... test_and_set_bit(); test_and_clear_bit(); test_and_change_bit(); ...
Trond
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