linux-kernel - Re: [PATCH] virtio: use smp_load_acquire/smp_store

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Date:	Thu, 17 Dec 2015 11:58:27 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	Jason Wang <jasowang@...hat.com>, qemu-devel@...gnu.org
Subject: Re: [PATCH] virtio: use smp_load_acquire/smp_store_release

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;


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