Re: What does atomic_read actually do?

["Joseph Seigh" <jseigh_02@xemaps.com>]

On Sat, 18 Dec 2004 19:34:27 +0100, Arjan van de Ven <arjan@infradead.org>  
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
> atomic_t.
>
> 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