On Thursday 09 August 2007 02:15:33 Andi Kleen wrote: > On Wed, Aug 08, 2007 at 05:08:44PM -0400, Chris Snook wrote: > > Heiko Carstens wrote: > > >On Wed, Aug 08, 2007 at 03:21:31AM -0700, David Miller wrote: > > >>From: Heiko Carstens <[EMAIL PROTECTED]> > > >>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 also think readding volatile makes sense. An alternative would be > to stick an rmb() into atomic_read() -- that would also stop speculative > reads. > Disadvantage is that it clobbers all memory, not just the specific value. > > But you really have to complain to Linus (cc'ed). He came up > with the volatile removale change iirc.
Isn't it possible through some inline assembly trick that only a certain variable has to be reloaded? So we could define something like that: #define reload_var(x) __asm__ __volatile__ (whatever, x) I don't know inline assembly that much, but isn't it possible with that to kind of "fake-touch" the variable, so the compiler must reload it (and only it) to make sure it's up to date? -- Greetings Michael. - To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
