[PATCH RFC v4 14/15] fuse: {io-uring} Prevent mount point hang on fuse-server termination

[Bernd Schubert <bschubert@ddn.com>]

When the fuse-server terminates while the fuse-client or kernel
still has queued URING_CMDs, these commands retain references
to the struct file used by the fuse connection. This prevents
fuse_dev_release() from being invoked, resulting in a hung mount
point.

This patch addresses the issue by making queued URING_CMDs
cancelable, allowing fuse_dev_release() to proceed as expected
and preventing the mount point from hanging.

Signed-off-by: Bernd Schubert <bschubert@ddn.com>
---
 fs/fuse/dev_uring.c | 62 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 62 insertions(+)

diff --git a/fs/fuse/dev_uring.c b/fs/fuse/dev_uring.c
index 6632c9163b8a51c39e07258fea631cf9383ce538..5603831d490c64045ff402140c317019e69f8987 100644
--- a/fs/fuse/dev_uring.c
+++ b/fs/fuse/dev_uring.c
@@ -21,6 +21,7 @@ MODULE_PARM_DESC(enable_uring,
 
 struct fuse_uring_cmd_pdu {
 	struct fuse_ring_ent *ring_ent;
+	struct fuse_ring_queue *queue;
 };
 
 /*
@@ -374,6 +375,61 @@ void fuse_uring_stop_queues(struct fuse_ring *ring)
 	}
 }
 
+/*
+ * Handle IO_URING_F_CANCEL, typically should come on on daemon termination
+ */
+static void fuse_uring_cancel(struct io_uring_cmd *cmd,
+			      unsigned int issue_flags, struct fuse_conn *fc)
+{
+	struct fuse_uring_cmd_pdu *pdu = (struct fuse_uring_cmd_pdu *)cmd->pdu;
+	struct fuse_ring_queue *queue = pdu->queue;
+	struct fuse_ring_ent *ent;
+	bool found = false;
+	bool need_cmd_done = false;
+
+	spin_lock(&queue->lock);
+
+	/* XXX: This is cumbersome for large queues. */
+	list_for_each_entry(ent, &queue->ent_avail_queue, list) {
+		if (pdu->ring_ent == ent) {
+			found = true;
+			break;
+		}
+	}
+
+	if (!found) {
+		pr_info("qid=%d Did not find ent=%p", queue->qid, ent);
+		spin_unlock(&queue->lock);
+		return;
+	}
+
+	if (ent->state == FRRS_WAIT) {
+		ent->state = FRRS_USERSPACE;
+		list_move(&ent->list, &queue->ent_in_userspace);
+		need_cmd_done = true;
+	}
+	spin_unlock(&queue->lock);
+
+	if (need_cmd_done)
+		io_uring_cmd_done(cmd, -ENOTCONN, 0, issue_flags);
+
+	/*
+	 * releasing the last entry should trigger fuse_dev_release() if
+	 * the daemon was terminated
+	 */
+}
+
+static void fuse_uring_prepare_cancel(struct io_uring_cmd *cmd, int issue_flags,
+				      struct fuse_ring_ent *ring_ent)
+{
+	struct fuse_uring_cmd_pdu *pdu = (struct fuse_uring_cmd_pdu *)cmd->pdu;
+
+	pdu->ring_ent = ring_ent;
+	pdu->queue = ring_ent->queue;
+
+	io_uring_cmd_mark_cancelable(cmd, issue_flags);
+}
+
 /*
  * Checks for errors and stores it into the request
  */
@@ -902,6 +958,7 @@ static int fuse_uring_fetch(struct io_uring_cmd *cmd, unsigned int issue_flags,
 		goto err;
 
 	atomic_inc(&ring->queue_refs);
+	fuse_uring_prepare_cancel(cmd, issue_flags, ring_ent);
 	_fuse_uring_fetch(ring_ent, cmd, issue_flags);
 
 	return 0;
@@ -947,6 +1004,11 @@ int fuse_uring_cmd(struct io_uring_cmd *cmd, unsigned int issue_flags)
 	if (fc->aborted)
 		goto out;
 
+	if ((unlikely(issue_flags & IO_URING_F_CANCEL))) {
+		fuse_uring_cancel(cmd, issue_flags, fc);
+		return 0;
+	}
+
 	switch (cmd_op) {
 	case FUSE_URING_REQ_FETCH:
 		err = fuse_uring_fetch(cmd, issue_flags, fc);

-- 
2.43.0