On Thu, 2013-06-20 at 04:59 +0800, Liu Ping Fan wrote: > diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c > index fbabf99..28abe1e 100644 > --- a/hw/virtio/vhost.c > +++ b/hw/virtio/vhost.c > @@ -16,6 +16,7 @@ > #include <sys/ioctl.h> > #include "hw/virtio/vhost.h" > #include "hw/hw.h" > +#include "qemu/atomic.h" > #include "qemu/range.h" > #include <linux/vhost.h> > #include "exec/address-spaces.h" > @@ -47,11 +48,9 @@ static void vhost_dev_sync_region(struct vhost_dev *dev, > addr += VHOST_LOG_CHUNK; > continue; > } > - /* Data must be read atomically. We don't really > - * need the barrier semantics of __sync > - * builtins, but it's easier to use them than > - * roll our own. */ > - log = __sync_fetch_and_and(from, 0); > + /* Data must be read atomically. We don't really need barrier > semantics > + * but it's easier to use atomic_* than roll our own. */ > + log = atomic_xchg(from, 0);
If you really don't need any ordering guarantees / barriers here, then using a relaxed load should be fine. But my gut feeling tells me you probably do need some barriers; either you are "re-using" another barrier (and then the comment should probably point out which), or it must be a case where it's either fine to read any value someone (else) wrote or there's no concurrent store after all.
