Re: [PATCH] make atomic_t volatile on all architectures

[Herbert Xu <herbert@gondor.apana.org.au>]

Chris Snook <csnook@redhat.com> wrote:
> 
> 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

Such loops should always use something like cpu_relax() which comes
with a barrier.

> of all registers used in the loop, thus hurting performance instead of helping
> it, particularly on architectures where it's unnecessary.  Since we generally

Do you have an example of such a loop where performance is hurt by this?

The IPVS code that led to this patch was simply broken and has been
fixed to use cpu_relax().

Cheers,
-- 
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
-
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/