Re: [PATCH] make atomic_t volatile on all architectures

["Jesper Juhl" <jesper.juhl@gmail.com>]

On 09/08/2007, Chris Snook <csnook@redhat.com> wrote:
> From: Chris Snook <csnook@redhat.com>
>
> Some architectures currently do not declare the contents of an atomic_t to be
> volatile.  This causes confusion since atomic_read() might not actually read
> anything if an optimizing compiler re-uses a value stored in a register, which
> can break code that loops until something external changes the value of an
> atomic_t.  Avoiding such bugs requires using barrier(), which causes re-loads
> of all registers used in the loop, thus hurting performance instead of helping
> it, particularly on architectures where it's unnecessary.  Since we generally
> want to re-read the contents of an atomic variable on every access anyway,
> let's standardize the behavior across all architectures and avoid the
> performance and correctness problems of requiring the use of barrier() in
> loops that expect atomic_t variables to change externally.  This is relevant
> even on non-smp architectures, since drivers may use atomic operations in
> interrupt handlers.
>
> Signed-off-by: Chris Snook <csnook@redhat.com>
>

Hmm, I thought we were trying to move away from volatile since it is
very weakly defined and towards explicit barriers and locks...
Points to --> Documentation/volatile-considered-harmful.txt


-- 
Jesper Juhl <jesper.juhl@gmail.com>
Don't top-post  http://www.catb.org/~esr/jargon/html/T/top-post.html
Plain text mails only, please      http://www.expita.com/nomime.html
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/