Christoph Lameter wrote:
On Mon, 13 Aug 2007, Chris Snook wrote:
@@ -38,7 +45,7 @@
Next, we have:
- #define atomic_read(v) ((v)->counter)
+ #define atomic_read(v) (*(volatile int *)&(v)->counter)
which simply reads the current value of the counter.
volatile means that there is some vague notion of "read it now". But that
really does not exist. Instead we control visibility via barriers
(smp_wmb, smp_rmb). Would it not be best to not have volatile at all in
atomic operations and let the barriers do the work?
From my reply in the other thread...
But barriers force a flush of *everything* in scope, which we generally don't
want. On the other hand, we pretty much always want to flush atomic_*
operations. One way or another, we should be restricting the volatile behavior
to the thing that needs it. On most architectures, this patch set just moves
that from the declaration, where it is considered harmful, to the use, where it
is considered an occasional necessary evil.
If you really, *really* distrust the compiler that much, you shouldn't be using
barrier, since that uses volatile under the hood too. You should just go ahead
and implement the atomic operations in assembler, like Segher Boessenkool did
for powerpc in response to my previous patchset.
-- Chris
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