[PATCH] vhost-vdpa: log warning message if vhost_vdpa_remove gets blocked

From: Gautam Dawar <gdawar.xilinx@gmail.com>
To: unlisted-recipients:; (no To-header on input)
Cc: martinh@xilinx.com, hanand@xilinx.com, gdawar@xilinx.com,
	"Michael S. Tsirkin" <mst@redhat.com>,
	Jason Wang <jasowang@redhat.com>,
	kvm@vger.kernel.org, virtualization@lists.linux-foundation.org,
	netdev@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: [PATCH] vhost-vdpa: log warning message if vhost_vdpa_remove gets blocked
Date: Sun,  6 Jun 2021 18:59:09 +0530	[thread overview]
Message-ID: <20210606132909.177640-1-gdawar.xilinx@gmail.com> (raw)

From: Gautam Dawar <gdawar@xilinx.com>

If some module invokes vdpa_device_unregister (usually in the module
unload function) when the userspace app (eg. QEMU) which had opened
the vhost-vdpa character device is still running, vhost_vdpa_remove()
function will block indefinitely in call to wait_for_completion().

This causes the vdpa_device_unregister caller to hang and with a
usual side-effect of rmmod command not returning when this call
is in the module_exit function.

This patch converts the wait_for_completion call to its timeout based
counterpart (wait_for_completion_timeout) and also adds a warning
message to alert the user/administrator about this hang situation.

To eventually fix this problem, a mechanism will be required to let
vhost-vdpa module inform the userspace of this situation and
userspace will close the descriptor of vhost-vdpa char device.
This will enable vhost-vdpa to continue with graceful clean-up.

Signed-off-by: Gautam Dawar <gdawar@xilinx.com>
---
 drivers/vhost/vdpa.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/vhost/vdpa.c b/drivers/vhost/vdpa.c
index bfa4c6ef554e..572b64d09b06 100644
--- a/drivers/vhost/vdpa.c
+++ b/drivers/vhost/vdpa.c
@@ -1091,7 +1091,11 @@ static void vhost_vdpa_remove(struct vdpa_device *vdpa)
 		opened = atomic_cmpxchg(&v->opened, 0, 1);
 		if (!opened)
 			break;
-		wait_for_completion(&v->completion);
+		wait_for_completion_timeout(&v->completion,
+					    msecs_to_jiffies(1000));
+		dev_warn_ratelimited(&v->dev,
+				     "%s waiting for /dev/%s to be closed\n",
+				     __func__, dev_name(&v->dev));
 	} while (1);
 
 	put_device(&v->dev);
-- 
2.30.1

