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SubjectRe: [PATCH 1/24] make atomic_read() behave consistently on alpha
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.

-- Chris
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