Re: What does atomic_read actually do?

From: Joseph Seigh
Date: Mon Dec 20 2004 - 15:48:31 EST

On Mon, 20 Dec 2004 13:52:23 +0100, Andrea Arcangeli <andrea@xxxxxxx> wrote:

set_pte_atomic also requires atomicity in
asm-generic/pgtable.h:ptep_establish, but it's not even using volatile
and it's a 64bit pointer that we're writing to. We're relaying on the
compiler to do the right thing for us. I don't think it's a good idea
for the long run, but Benjamin on ppc64 rejected my suggestion to
rewrite set_pte_atomic in asm, so I doubt you'll be able to rewrite
atomic_read with asm either (because at least atomic_read is an int and
not a long pointer, and at least atomic_read is a volatile unlike

My point is that even before worrying about the theoretical correctness
of atomic_read, I would suggest to worry about set_pte_atomic first,
which is a lot more likely to break if something. The compiler may truly
execute two separate writes if power of 2 bitshifts are involved, as the
optimal compilation of the C source.

Actually, I'm programming out in user space. I was looking at the atomic.h
stuff to see how much would be usable. But I have to look at reading
and writing pointers atomically also since that is heavily used in lock-free
programming, e.g. RCU for preemptive user threads which I have a couple of
implementations for. I remember a discussion a while back about dependent
load memory ordering (alpha doesn't have it) for RCU but I don't recall any
discussion of the requirement for gcc to do atomic loads and stores. I guess
it's just assumed.

It would be nice to have macros tied to the linux ones, ie. check the current
linux versions when porting them. But I can always use asm if I have to. I'll
probably have to modify the conditions of use to say "no warranty is implied
and we really do mean it" and add a compiler flag to select between the safe asm
version and the "Do you feel lucky?" C macro version.

Joe Seigh

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