Messages in this thread | | | Date | Thu, 09 Aug 2007 08:40:43 -0400 | From | Chris Snook <> | Subject | Re: [patch] ipvs: force read of atomic_t in while loop |
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Michael Buesch wrote: > 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 <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 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?
We can do it in C, like this:
-#define atomic_read(v) ((v)->counter) +#define atomic_read(v) (*(volatile int *)&(v)->counter)
By casting it volatile at the precise piece of code where we want to guarantee a read from memory, there's little risk of the compiler getting creative in its interpretation of the code.
Stay tuned for the patch set...
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