Messages in this thread | | | Date | Wed, 8 Aug 2007 14:31:15 -0700 | From | Andrew Morton <> | Subject | Re: [patch] ipvs: force read of atomic_t in while loop |
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On Wed, 08 Aug 2007 17:08:44 -0400 Chris Snook <csnook@redhat.com> wrote:
> Heiko Carstens wrote: > > On Wed, Aug 08, 2007 at 03:21:31AM -0700, David Miller wrote: > >> From: Heiko Carstens <heiko.carstens@de.ibm.com> > >> Date: Wed, 8 Aug 2007 11:33:00 +0200 > >> > >>> Just saw this while grepping for atomic_reads in a while loops. > >>> Maybe we should re-add the volatile to atomic_t. Not sure. > >> I think whatever the choice, it should be done consistently > >> on every architecture. > >> > >> It's just asking for trouble if your arch does it differently from > >> every other. > > > > Well..currently it's i386/x86_64 and s390 which have no volatile > > in atomic_t. And yes, of course I agree it should be consistent > > across all architectures. But it isn't. > > Based on recent discussion, it's pretty clear that there's a lot of > confusion about this. A lot of people (myself included, until I thought > about it long and hard) will reasonably assume that calling > atomic_read() will actually read the value from memory. Leaving out the > volatile declaration seems like a pessimization to me. If you force > people to use barrier() everywhere they're working with atomic_t, it > will force re-reads of all the non-atomic data in use as well, which > will cause more memory fetches of things that generally don't need > barrier(). That and it's a bug waiting to happen. > > Andi -- your thoughts on the matter?
I'm not Andi, but this not-Andi thinks that permitting the compiler to cache the results of atomic_read() is dumb.
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