linux-kernel - Re: [PATCH] ptr

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Date:   Wed, 6 Dec 2017 11:21:54 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     linux-kernel@...r.kernel.org,
        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 10:53, Michael S. Tsirkin wrote:
> On Wed, Dec 06, 2017 at 10:31:39AM +0800, Jason Wang wrote:
>>
>> 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
> Batch consumers only do smp_read_barrier_depends which is free on
> non-alpha. I suggest we do the simple thing for stable and reserve
> optimizations for later.
>

Right.

Acked-by: Jason Wang <jasowang@...hat.com>