Re: What does atomic_read actually do?

From: Joseph Seigh
Date: Sat Dec 18 2004 - 14:18:23 EST

On Sat, 18 Dec 2004 19:34:27 +0100, Arjan van de Ven <arjan@xxxxxxxxxxxxx> wrote:

On Sat, 2004-12-18 at 11:23 -0500, Joseph Seigh wrote:
It doesn't do anything that would actually guarantee that the fetch from
memory would be atomic as far as I can see, at least in the x86 version.

define atomic....

what linux atomics guarantee you is that you either "see" the old or the
new value if you use atomic_* as the sole accessor API, with the
footnote that this only holds if you don't forcefully misalign the

if you want ordering guarantees on top... you need to use explicit
bariers for that (wmb/rmb and friends).

For the "no inbetween" rule, doing the read the way x86 does works on
x86, since x86 makes sure that on the write side, no intermediate
results become visible.

I mean atomic in the either old or new sense. I'm wondering what guarantees
the atomicity. Not the C standard. I can see the gcc compiler uses a MOV
instruction to load the atomic_t from memory which is guaranteed atomic by
the architecture if aligned properly. But gcc does that for any old int
as far as I can see, so why use atomic_read?

Joe Seigh

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