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

From: Chris Snook
Date: Thu Aug 09 2007 - 15:32:20 EST


Paul E. McKenney wrote:
On Thu, Aug 09, 2007 at 02:13:52PM -0400, Chris Snook wrote:
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.

This can also happen when using per-CPU variables. And there are a
number of per-CPU variables that are either atomic themselves or are
structures containing atomic fields.

Accessing per-CPU variables in this fashion reliably already requires a suitable smp/non-smp read/write memory barrier. I maintain that if we break anything with this change, it was really already broken, if less obviously. Can you give a real or synthetic example of legitimate code that could break?

-- Chris
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