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Commit a8ceb5db authored by Michael S. Tsirkin's avatar Michael S. Tsirkin Committed by David S. Miller
Browse files

ptr_ring: add barriers 



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: default avatarGeorge Cherian <george.cherian@cavium.com>
Suggested-by: default avatarJason Wang <jasowang@redhat.com>
Signed-off-by: default avatarMichael S. Tsirkin <mst@redhat.com>
Acked-by: default avatarJason Wang <jasowang@redhat.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent f0f1d016

include/linux/ptr_ring.h

+9 −0
Original line number Diff line number Diff line
@@ -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;

}