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.
Thanx, Paul
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