Messages in this thread | | | Date | Thu, 9 Aug 2007 08:04:45 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH 1/24] make atomic_read() behave consistently on alpha |
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On Thu, Aug 09, 2007 at 10:53:14AM -0400, Chris Snook wrote: > Paul E. McKenney wrote: > >Why not the same access-once semantics for atomic_set() as > >for atomic_read()? As this patch stands, it might introduce > >architecture-specific compiler-induced bugs due to the fact that > >atomic_set() used to imply volatile behavior but no longer does. > > When we make the volatile cast in atomic_read(), we're casting an rvalue to > volatile. This unambiguously tells the compiler that we want to re-load > that register from memory. What's "volatile behavior" for an lvalue?
I was absolutely -not- suggesting volatile behavior for lvalues.
Instead, I am asking for volatile behavior from an -rvalue-. In the case of atomic_read(), it is the atomic_t being read from. In the case of atomic_set(), it is the atomic_t being written to. As suggested in my previous email:
#define atomic_set(v,i) ((*(volatile int *)&(v)->counter) = (i)) #define atomic64_set(v,i) ((*(volatile long *)&(v)->counter) = (i))
Again, the architectures that used to have their "counter" declared as volatile will lose volatile semantics on atomic_set() with your patch, which might result in bugs due to overly imaginative compiler optimizations. The above would prevent any such bugs from appearing.
> A > write to an lvalue already implies an eventual write to memory, so this > would be a no-op. Maybe you'll write to the register a few times before > flushing it to memory, but it will happen eventually. With an rvalue, > there's no guarantee that it will *ever* load from memory, which is what > volatile fixes. > > I think what you have in mind is LOCK_PREFIX behavior, which is not the > purpose of atomic_set. We use LOCK_PREFIX in the inline assembly for the > atomic_* operations that read, modify, and write a value, only because it > is necessary to perform that entire transaction atomically.
No LOCK_PREFIX, thank you!!! I just want to make sure that the compiler doesn't push the store down out of a loop, split the store, allow the store to happen twice (e.g., to allow different code paths to be merged), and all the other tricks that the C standard permits compilers to pull.
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