[dpdk-dev] [PATCH 1/2] vhost: fix deadlock on IOTLB miss

[Maxime Coquelin <maxime.coquelin@redhat.com>]

An optimization was done to only take the iotlb cache lock
once per packet burst instead of once per IOVA translation.

With this, IOTLB miss requests are sent to Qemu with the lock
held, which can cause a deadlock if the socket buffer is full,
and if Qemu is waiting for an IOTLB update to be done.

Holding the lock is not necessary when sending an IOTLB miss
request, as it is not manipulating the IOTLB cache list, which
the lock protects. Let's just release it while sending the
IOTLB miss.

Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
---
 lib/librte_vhost/vhost.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/lib/librte_vhost/vhost.c b/lib/librte_vhost/vhost.c
index 54a1864eb..4f8b73a09 100644
--- a/lib/librte_vhost/vhost.c
+++ b/lib/librte_vhost/vhost.c
@@ -55,6 +55,7 @@
 
 struct virtio_net *vhost_devices[MAX_VHOST_DEVICE];
 
+/* Called with iotlb_lock read-locked */
 uint64_t
 __vhost_iova_to_vva(struct virtio_net *dev, struct vhost_virtqueue *vq,
 		    uint64_t iova, uint64_t size, uint8_t perm)
@@ -71,8 +72,19 @@ __vhost_iova_to_vva(struct virtio_net *dev, struct vhost_virtqueue *vq,
 		return vva;
 
 	if (!vhost_user_iotlb_pending_miss(vq, iova + tmp_size, perm)) {
+		/*
+		 * iotlb_lock is read-locked for a full burst,
+		 * but it only protects the iotlb cache.
+		 * In case of IOTLB miss, we might block on the socket,
+		 * which could cause a deadlock with QEMU if an IOTLB update
+		 * is being handled. We can safely unlock here to avoid it.
+		 */
+		vhost_user_iotlb_rd_unlock(vq);
+
 		vhost_user_iotlb_pending_insert(vq, iova + tmp_size, perm);
 		vhost_user_iotlb_miss(dev, iova + tmp_size, perm);
+
+		vhost_user_iotlb_rd_lock(vq);
 	}
 
 	return 0;
-- 
2.13.6