On Thu, Dec 17, 2015 at 11:58:27AM +0100, Peter Zijlstra wrote:
> 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.
Yes - it just so happens that READ barrier is enough for where I use it
for virtio.
I really just need virtio_load_acquire that fences reads,
I don't care what happens to writes.
Given the confusion, maybe virtio_load_acquire is a bad name though.
Donnu what a good name would be.
virtio_load_acquire_rmb and virtio_store_release_wmb?
> So your #else branch should look something like:
>
> var = READ_ONCE(*p);
> dma_mb();
> return var;
>
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