Re: [Qemu-devel] [PATCH] virtio: use smp_load_acquire/smp_store_release

[Peter Zijlstra]

On Thu, Dec 17, 2015 at 12:29:03PM +0200, Michael S. Tsirkin wrote:
> +static inline __virtio16 virtio_load_acquire(bool weak_barriers, __virtio16 
> *p)
> +{
> +     if (!weak_barriers) {
> +             rmb();
> +             return READ_ONCE(*p);
> +     }
> +#ifdef CONFIG_SMP
> +     return smp_load_acquire(p);
> +#else
> +     dma_rmb();
> +     return READ_ONCE(*p);
> +#endif
> +}

This too is wrong. Look for example at arm.

dma_rmb() is dmb(osh), while the smp_mb() used by smp_load_acquire() is
dmb(ish). They order completely different types of memory accesses.

Also, load_acquire() is first load, then barrier, and an ACQUIRE barrier
at that, not a READ barrier.

So your #else branch should look something like:

        var = READ_ONCE(*p);
        dma_mb();
        return var;