Re: [Qemu-devel] [PATCH v2] atomics: add explicit compiler fence in __atomic memory barriers

[Stefan Hajnoczi]

On Wed, Jun 03, 2015 at 02:34:51PM +0200, Paolo Bonzini wrote:
> __atomic_thread_fence does not include a compiler barrier; in the
> C++11 memory model, fences take effect in combination with other
> atomic operations.  GCC implements this by making __atomic_load and
> __atomic_store access memory as if the pointer was volatile, and
> leaves no trace whatsoever of acquire and release fences in the
> compiler's intermediate representation.
> 
> In QEMU, we want memory barriers to act on all memory, but at the same
> time we would like to use __atomic_thread_fence for portability reasons.
> Add compiler barriers manually around the __atomic_thread_fence.
> 
> Thanks to Kevin Wolf for the analysis.
> 
> Signed-off-by: Paolo Bonzini <address@hidden>
> ---
>         v1->v2: improved comment [Peter]
> ---
>  include/qemu/atomic.h | 14 +++++++++++---
>  1 file changed, 11 insertions(+), 3 deletions(-)

Reviewed-by: Stefan Hajnoczi <address@hidden>

pgp6As_xMS2WY.pgp
Description: PGP signature