Messages in this thread | | | Date | Tue, 14 Aug 2007 15:56:51 -0700 (PDT) | From | Christoph Lameter <> | Subject | Re: [PATCH 1/23] document preferred use of volatile with atomic_t |
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On Tue, 14 Aug 2007, Chris Snook wrote:
> > 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.
From my reply on the other thread:
Maybe we need two read functions? One volatile, one not?
The atomic_read()s that I have in slub really do not care about when the variables are read. And if volatile creates overhead then I rather not have it.
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