Messages in this thread | | | Date | Thu, 09 Aug 2007 14:13:52 -0400 | From | Chris Snook <> | Subject | Re: [PATCH 1/24] make atomic_read() behave consistently on alpha |
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Paul E. McKenney wrote: > On Thu, Aug 09, 2007 at 01:14:35PM -0400, Chris Snook wrote: >> If you're depending on volatile writes >> being visible to other CPUs, you're screwed either way, because the CPU can >> hold that data in cache as long as it wants before it writes it to memory. >> When this finally does happen, it will happen atomically, which is all that >> atomic_set guarantees. If you need to guarantee that the value is written >> to memory at a particular time in your execution sequence, you either have >> to read it from memory to force the compiler to store it first (and a >> volatile cast in atomic_read will suffice for this) or you have to use >> LOCK_PREFIX instructions which will invalidate remote cache lines >> containing the same variable. This patch doesn't change either of these >> cases. > > The case that it -can- change is interactions with interrupt handlers. > And NMI/SMI handlers, for that matter.
You have a point here, but only if you can guarantee that the interrupt handler is running on a processor sharing the cache that has the not-yet-written volatile value. That implies a strictly non-SMP architecture. At the moment, none of those have volatile in their declaration of atomic_t, so this patch can't break any of them.
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