On Thu, Aug 09, 2007 at 10:53:14AM -0400, Chris Snook wrote:

Paul E. McKenney wrote:

quoted

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