Hi Maxime,

-----Original Message-----
From: Maxime Coquelin <redacted>
Sent: Tuesday, April 13, 2021 7:33 PM
To: Hu, Jiayu <redacted>; dev@dpdk.org
Cc: Xia, Chenbo <redacted>; Wang, Yinan
[off-list ref]; Pai G, Sunil [off-list ref]; Jiang, Cheng1
[off-list ref]
Subject: Re: [PATCH v2 3/4] vhost: avoid deadlock on async register



On 4/2/21 3:04 PM, Jiayu Hu wrote:

quoted

Users can register async copy device in vring_state_changed(),
when vhost queue is enabled. However, a deadlock occurs inside
rte_vhost_async_channel_register(), if

VHOST_USER_F_PROTOCOL_FEATURES

quoted

is not supported, as vhost_user_msg_handler() takes vq->access_lock
before calling vhost_user_set_vring_kick().

This patch avoids async register deadlock by removing calling
vring_state_changed() in vhost_user_set_vring_kick(). It's safe
as vhost_user_msg_handler() will call vring_state_changed() anyway.

Signed-off-by: Jiayu Hu <redacted>
---
 lib/librte_vhost/vhost_user.c | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/lib/librte_vhost/vhost_user.c b/lib/librte_vhost/vhost_user.c
index 44c0452..8f0eba6 100644
--- a/lib/librte_vhost/vhost_user.c
+++ b/lib/librte_vhost/vhost_user.c

@@ -1918,9 +1918,6 @@ vhost_user_set_vring_kick(struct virtio_net

**pdev, struct VhostUserMsg *msg,

quoted

 	 */
 	if (!(dev->features & (1ULL <<

VHOST_USER_F_PROTOCOL_FEATURES))) {

quoted

 		vq->enabled = true;
-		if (dev->notify_ops->vring_state_changed)
-			dev->notify_ops->vring_state_changed(
-				dev->vid, file.index, 1);
 	}

 	if (vq->ready) {

As replied earlier on v1, I agree the call to vring_state_changed here
is not needed. But it might not be enough, there are other cases where
you could have issues.

vhost_user_notify_queue_state() can be called in three cases:
1. when vq ready status changes, vhost_user_msg_handler() calls it to notify
backend. But vhost_user_msg_handler() doesn't take lock before calling it.
So in this case, no deadlock occurs in async register.

2. if vq->ready is true, vhost_user_set_vring_call() calls it to notify backend
vq is not enabled. Although vhost_user_set_vring_call() is protected by lock,
async register is called only if vq is enabled, so async register will not be called
in this case.

3. If vq->ready is true, vhost_user_set_vring_kick() calls it to notify backend
vq is not enabled. Same as #2, async register is called only when vq is enabled.
Even if vhost_user_set_vring_kick() is protected by lock, there is no deadlock in
async register, as it will not be called in this case.

In summary,  I think there is no deadlock issue in async register if we
can remove calling vring_state_change() in vhost_user_set_vring_kick().

Please add stable and Fixes tag.

Do you suggest to make the patch as a fix for 8639d54563a
("vhost: introduce async enqueue registration API")? But the
thing is that code removed in this patch is not introduced
by this commit.

Thanks,
Jiayu

Thanks,
Maxime