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Date:   Wed, 6 Dec 2017 10:31:39 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>, linux-kernel@...r.kernel.org
Cc:     George Cherian <george.cherian@...ium.com>, davem@...emloft.net,
        edumazet@...gle.com, netdev@...r.kernel.org,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH] ptr_ring: add barriers



On 2017年12月06日 03:29, Michael S. Tsirkin wrote:
> Users of ptr_ring expect that it's safe to give the
> data structure a pointer and have it be available
> to consumers, but that actually requires an smb_wmb
> or a stronger barrier.
>
> In absence of such barriers and on architectures that reorder writes,
> consumer might read an un=initialized value from an skb pointer stored
> in the skb array.  This was observed causing crashes.
>
> To fix, add memory barriers.  The barrier we use is a wmb, the
> assumption being that producers do not need to read the value so we do
> not need to order these reads.
>
> Reported-by: George Cherian <george.cherian@...ium.com>
> Suggested-by: Jason Wang <jasowang@...hat.com>
> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> ---
>
> George, could you pls report whether this patch fixes
> the issue for you?
>
> This seems to be needed in stable as well.
>
>
>
>
>   include/linux/ptr_ring.h | 9 +++++++++
>   1 file changed, 9 insertions(+)
>
> diff --git a/include/linux/ptr_ring.h b/include/linux/ptr_ring.h
> index 37b4bb2..6866df4 100644
> --- a/include/linux/ptr_ring.h
> +++ b/include/linux/ptr_ring.h
> @@ -101,12 +101,18 @@ static inline bool ptr_ring_full_bh(struct ptr_ring *r)
>   
>   /* Note: callers invoking this in a loop must use a compiler barrier,
>    * for example cpu_relax(). Callers must hold producer_lock.
> + * Callers are responsible for making sure pointer that is being queued
> + * points to a valid data.
>    */
>   static inline int __ptr_ring_produce(struct ptr_ring *r, void *ptr)
>   {
>   	if (unlikely(!r->size) || r->queue[r->producer])
>   		return -ENOSPC;
>   
> +	/* Make sure the pointer we are storing points to a valid data. */
> +	/* Pairs with smp_read_barrier_depends in __ptr_ring_consume. */
> +	smp_wmb();
> +
>   	r->queue[r->producer++] = ptr;
>   	if (unlikely(r->producer >= r->size))
>   		r->producer = 0;
> @@ -275,6 +281,9 @@ static inline void *__ptr_ring_consume(struct ptr_ring *r)
>   	if (ptr)
>   		__ptr_ring_discard_one(r);
>   
> +	/* Make sure anyone accessing data through the pointer is up to date. */
> +	/* Pairs with smp_wmb in __ptr_ring_produce. */
> +	smp_read_barrier_depends();
>   	return ptr;
>   }
>   

I was thinking whether or not it's better to move those to the callers. 
Then we can save lots of barriers in e.g batch consuming.

Thanks

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