Patch "vhost: Release worker mutex during flushes" has been added to the 6.6-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    vhost: Release worker mutex during flushes

to the 6.6-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     vhost-release-worker-mutex-during-flushes.patch
and it can be found in the queue-6.6 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 2a02a05871008c96f9a4078b3c575d28e9c5bedf
Author: Mike Christie <michael.christie@xxxxxxxxxx>
Date:   Fri Mar 15 19:47:05 2024 -0500

    vhost: Release worker mutex during flushes
    
    [ Upstream commit ba704ff4e142fd3cfaf3379dd3b3b946754e06e3 ]
    
    In the next patches where the worker can be killed while in use, we
    need to be able to take the worker mutex and kill queued works for
    new IO and flushes, and set some new flags to prevent new
    __vhost_vq_attach_worker calls from swapping in/out killed workers.
    
    If we are holding the worker mutex during a flush and the flush's work
    is still in the queue, the worker code that will handle the SIGKILL
    cleanup won't be able to take the mutex and perform it's cleanup. So
    this patch has us drop the worker mutex while waiting for the flush
    to complete.
    
    Signed-off-by: Mike Christie <michael.christie@xxxxxxxxxx>
    Message-Id: <20240316004707.45557-8-michael.christie@xxxxxxxxxx>
    Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 67bd947cc556d..069c8a23bff9e 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -276,21 +276,36 @@ void vhost_vq_flush(struct vhost_virtqueue *vq)
 EXPORT_SYMBOL_GPL(vhost_vq_flush);
 
 /**
- * vhost_worker_flush - flush a worker
+ * __vhost_worker_flush - flush a worker
  * @worker: worker to flush
  *
- * This does not use RCU to protect the worker, so the device or worker
- * mutex must be held.
+ * The worker's flush_mutex must be held.
  */
-static void vhost_worker_flush(struct vhost_worker *worker)
+static void __vhost_worker_flush(struct vhost_worker *worker)
 {
 	struct vhost_flush_struct flush;
 
+	if (!worker->attachment_cnt)
+		return;
+
 	init_completion(&flush.wait_event);
 	vhost_work_init(&flush.work, vhost_flush_work);
 
 	vhost_worker_queue(worker, &flush.work);
+	/*
+	 * Drop mutex in case our worker is killed and it needs to take the
+	 * mutex to force cleanup.
+	 */
+	mutex_unlock(&worker->mutex);
 	wait_for_completion(&flush.wait_event);
+	mutex_lock(&worker->mutex);
+}
+
+static void vhost_worker_flush(struct vhost_worker *worker)
+{
+	mutex_lock(&worker->mutex);
+	__vhost_worker_flush(worker);
+	mutex_unlock(&worker->mutex);
 }
 
 void vhost_dev_flush(struct vhost_dev *dev)
@@ -298,15 +313,8 @@ void vhost_dev_flush(struct vhost_dev *dev)
 	struct vhost_worker *worker;
 	unsigned long i;
 
-	xa_for_each(&dev->worker_xa, i, worker) {
-		mutex_lock(&worker->mutex);
-		if (!worker->attachment_cnt) {
-			mutex_unlock(&worker->mutex);
-			continue;
-		}
+	xa_for_each(&dev->worker_xa, i, worker)
 		vhost_worker_flush(worker);
-		mutex_unlock(&worker->mutex);
-	}
 }
 EXPORT_SYMBOL_GPL(vhost_dev_flush);
 
@@ -685,7 +693,6 @@ static void __vhost_vq_attach_worker(struct vhost_virtqueue *vq,
 	 * device wide flushes which doesn't use RCU for execution.
 	 */
 	mutex_lock(&old_worker->mutex);
-	old_worker->attachment_cnt--;
 	/*
 	 * We don't want to call synchronize_rcu for every vq during setup
 	 * because it will slow down VM startup. If we haven't done
@@ -696,6 +703,8 @@ static void __vhost_vq_attach_worker(struct vhost_virtqueue *vq,
 	mutex_lock(&vq->mutex);
 	if (!vhost_vq_get_backend(vq) && !vq->kick) {
 		mutex_unlock(&vq->mutex);
+
+		old_worker->attachment_cnt--;
 		mutex_unlock(&old_worker->mutex);
 		/*
 		 * vsock can queue anytime after VHOST_VSOCK_SET_GUEST_CID.
@@ -711,7 +720,8 @@ static void __vhost_vq_attach_worker(struct vhost_virtqueue *vq,
 	/* Make sure new vq queue/flush/poll calls see the new worker */
 	synchronize_rcu();
 	/* Make sure whatever was queued gets run */
-	vhost_worker_flush(old_worker);
+	__vhost_worker_flush(old_worker);
+	old_worker->attachment_cnt--;
 	mutex_unlock(&old_worker->mutex);
 }
 
@@ -764,6 +774,12 @@ static int vhost_free_worker(struct vhost_dev *dev,
 		mutex_unlock(&worker->mutex);
 		return -EBUSY;
 	}
+	/*
+	 * A flush might have raced and snuck in before attachment_cnt was set
+	 * to zero. Make sure flushes are flushed from the queue before
+	 * freeing.
+	 */
+	__vhost_worker_flush(worker);
 	mutex_unlock(&worker->mutex);
 
 	vhost_worker_destroy(dev, worker);