[PATCH 4/4] virtiofs: Support blocking posix locks (fcntl(F_SETLKW))

[Vivek Goyal <vgoyal@redhat.com>]

As of now we don't support blocking variant of posix locks and daemon returns
-EOPNOTSUPP. Reason being that it can lead to deadlocks. Virtqueue size is
limited and it is possible we fill virtqueue with all the requests of
fcntl(F_SETLKW) and wait for reply. And later a subsequent unlock request
can't make progress because virtqueue is full. And that means F_SETLKW can't
make progress and we are deadlocked.

Use notification queue to solve this problem. After submitting lock request
device will send a reply asking requester to wait. Once lock is available,
requester will get a notification saying locking is available. That way
we don't keep the request virtueue busy while we are waiting for lock
and further unlock requests can make progress.

When we get a reply in response to lock request, we need a way to know if
we need to wait for notification or not. I have overloaded the
fuse_out_header->error field. If value is ->error is 1, that's a signal
to caller to wait for lock notification.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
---
 fs/fuse/virtio_fs.c       | 78 ++++++++++++++++++++++++++++++++++++++-
 include/uapi/linux/fuse.h |  7 ++++
 2 files changed, 84 insertions(+), 1 deletion(-)

diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c
index 21d8d9d7d317..8aa9fc996556 100644
--- a/fs/fuse/virtio_fs.c
+++ b/fs/fuse/virtio_fs.c
@@ -35,6 +35,7 @@ struct virtio_fs_vq {
 	struct work_struct done_work;
 	struct list_head queued_reqs;
 	struct list_head end_reqs;	/* End these requests */
+	struct list_head wait_reqs;	/* requests waiting for notification */
 	struct virtio_fs_notify_node *notify_nodes;
 	struct list_head notify_reqs;	/* List for queuing notify requests */
 	struct delayed_work dispatch_work;
@@ -85,7 +86,6 @@ struct virtio_fs_notify_node {
 
 static int virtio_fs_enqueue_all_notify(struct virtio_fs_vq *fsvq);
 
-
 static inline struct virtio_fs_vq *vq_to_fsvq(struct virtqueue *vq)
 {
 	struct virtio_fs *fs = vq->vdev->priv;
@@ -513,13 +513,75 @@ static int virtio_fs_enqueue_all_notify(struct virtio_fs_vq *fsvq)
 	return 0;
 }
 
+static int notify_complete_waiting_req(struct virtio_fs *vfs,
+				       struct fuse_notify_lock_out *out_args)
+{
+	struct virtio_fs_vq *fsvq = &vfs->vqs[VQ_REQUEST];
+	struct fuse_req *req, *next;
+	bool found = false;
+	struct fuse_conn *fc = fsvq->fud->fc;
+
+	/* Find waiting request with the unique number and end it */
+	spin_lock(&fsvq->lock);
+		list_for_each_entry_safe(req, next, &fsvq->wait_reqs, list) {
+			if (req->in.h.unique == out_args->id) {
+				list_del_init(&req->list);
+				clear_bit(FR_SENT, &req->flags);
+				/* Transfer error code from notify */
+				req->out.h.error = out_args->error;
+				found = true;
+				break;
+			}
+		}
+	spin_unlock(&fsvq->lock);
+
+	/*
+	 * TODO: It is possible that some re-ordering happens in notify
+	 * comes before request is complete. Deal with it.
+	 */
+	if (found) {
+		fuse_request_end(fc, req);
+		spin_lock(&fsvq->lock);
+		dec_in_flight_req(fsvq);
+		spin_unlock(&fsvq->lock);
+	} else
+		pr_debug("virtio-fs: Did not find waiting request with"
+		       " unique=0x%llx\n", out_args->id);
+
+	return 0;
+}
+
+static int virtio_fs_handle_notify(struct virtio_fs *vfs,
+				   struct virtio_fs_notify *notify)
+{
+	int ret = 0;
+	struct fuse_out_header *oh = &notify->out_hdr;
+	struct fuse_notify_lock_out *lo;
+
+	/*
+	 * For notifications, oh.unique is 0 and oh->error contains code
+	 * for which notification as arrived.
+	 */
+	switch(oh->error) {
+	case FUSE_NOTIFY_LOCK:
+		lo = (struct fuse_notify_lock_out *) &notify->outarg;
+		notify_complete_waiting_req(vfs, lo);
+		break;
+	default:
+		printk("virtio-fs: Unexpected notification %d\n", oh->error);
+	}
+	return ret;
+}
+
 static void virtio_fs_notify_done_work(struct work_struct *work)
 {
 	struct virtio_fs_vq *fsvq = container_of(work, struct virtio_fs_vq,
 						 done_work);
 	struct virtqueue *vq = fsvq->vq;
+	struct virtio_fs *vfs = vq->vdev->priv;
 	LIST_HEAD(reqs);
 	struct virtio_fs_notify_node *notify, *next;
+	struct fuse_out_header *oh;
 
 	spin_lock(&fsvq->lock);
 	do {
@@ -535,6 +597,10 @@ static void virtio_fs_notify_done_work(struct work_struct *work)
 
 	/* Process notify */
 	list_for_each_entry_safe(notify, next, &reqs, list) {
+		oh = &notify->notify.out_hdr;
+		WARN_ON(oh->unique);
+		/* Handle notification */
+		virtio_fs_handle_notify(vfs, &notify->notify);
 		spin_lock(&fsvq->lock);
 		dec_in_flight_req(fsvq);
 		list_del_init(&notify->list);
@@ -656,6 +722,15 @@ static void virtio_fs_requests_done_work(struct work_struct *work)
 		 * TODO verify that server properly follows FUSE protocol
 		 * (oh.uniq, oh.len)
 		 */
+		if (req->out.h.error == 1) {
+			/* Wait for notification to complete request */
+			list_del_init(&req->list);
+			spin_lock(&fsvq->lock);
+			list_add_tail(&req->list, &fsvq->wait_reqs);
+			spin_unlock(&fsvq->lock);
+			continue;
+		}
+
 		args = req->args;
 		copy_args_from_argbuf(args, req);
 
@@ -705,6 +780,7 @@ static int virtio_fs_init_vq(struct virtio_fs *fs, struct virtio_fs_vq *fsvq,
 	strncpy(fsvq->name, name, VQ_NAME_LEN);
 	spin_lock_init(&fsvq->lock);
 	INIT_LIST_HEAD(&fsvq->queued_reqs);
+	INIT_LIST_HEAD(&fsvq->wait_reqs);
 	INIT_LIST_HEAD(&fsvq->end_reqs);
 	INIT_LIST_HEAD(&fsvq->notify_reqs);
 	init_completion(&fsvq->in_flight_zero);
diff --git a/include/uapi/linux/fuse.h b/include/uapi/linux/fuse.h
index 373cada89815..45f0c4efec8e 100644
--- a/include/uapi/linux/fuse.h
+++ b/include/uapi/linux/fuse.h
@@ -481,6 +481,7 @@ enum fuse_notify_code {
 	FUSE_NOTIFY_STORE = 4,
 	FUSE_NOTIFY_RETRIEVE = 5,
 	FUSE_NOTIFY_DELETE = 6,
+	FUSE_NOTIFY_LOCK = 7,
 	FUSE_NOTIFY_CODE_MAX,
 };
 
@@ -868,6 +869,12 @@ struct fuse_notify_retrieve_in {
 	uint64_t	dummy4;
 };
 
+struct fuse_notify_lock_out {
+	uint64_t	id;
+	int32_t		error;
+	int32_t		padding;
+};
+
 /* Device ioctls: */
 #define FUSE_DEV_IOC_CLONE	_IOR(229, 0, uint32_t)
 
-- 
2.20.1