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Re: [Qemu-devel] [PATCH] virtio_ring: use smp_store_mb


From: Peter Zijlstra
Subject: Re: [Qemu-devel] [PATCH] virtio_ring: use smp_store_mb
Date: Thu, 17 Dec 2015 11:52:38 +0100
User-agent: Mutt/1.5.21 (2012-12-30)

On Thu, Dec 17, 2015 at 12:32:53PM +0200, Michael S. Tsirkin wrote:
> +static inline void virtio_store_mb(bool weak_barriers,
> +                                __virtio16 *p, __virtio16 v)
> +{
> +#ifdef CONFIG_SMP
> +     if (weak_barriers)
> +             smp_store_mb(*p, v);
> +     else
> +#endif
> +     {
> +             WRITE_ONCE(*p, v);
> +             mb();
> +     }
> +}

This is a different barrier depending on SMP, that seems wrong.

smp_mb(), as (should be) used by smp_store_mb() does not provide a
barrier against IO. mb() otoh does.

Since this is virtIO I would expect you always want mb().



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