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From:   Vivek Goyal <vgoyal@redhat.com>
To:     virtio-fs@redhat.com, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org
Cc:     vgoyal@redhat.com, stefanha@redhat.com, dgilbert@redhat.com,
        miklos@szeredi.hu
Subject: [PATCH 4/4] virtiofs: Support blocking posix locks (fcntl(F_SETLKW))
Date:   Fri, 15 Nov 2019 15:57:05 -0500
Message-Id: <20191115205705.2046-5-vgoyal@redhat.com>
In-Reply-To: <20191115205705.2046-1-vgoyal@redhat.com>
References: <20191115205705.2046-1-vgoyal@redhat.com>
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Sender: linux-kernel-owner@vger.kernel.org
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As of now we don't support blocking variant of posix locks and daemon retur=
ns
-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 {
 =09struct work_struct done_work;
 =09struct list_head queued_reqs;
 =09struct list_head end_reqs;=09/* End these requests */
+=09struct list_head wait_reqs;=09/* requests waiting for notification */
 =09struct virtio_fs_notify_node *notify_nodes;
 =09struct list_head notify_reqs;=09/* List for queuing notify requests */
 =09struct delayed_work dispatch_work;
@@ -85,7 +86,6 @@ struct virtio_fs_notify_node {
=20
 static int virtio_fs_enqueue_all_notify(struct virtio_fs_vq *fsvq);
=20
-
 static inline struct virtio_fs_vq *vq_to_fsvq(struct virtqueue *vq)
 {
 =09struct virtio_fs *fs =3D vq->vdev->priv;
@@ -513,13 +513,75 @@ static int virtio_fs_enqueue_all_notify(struct virtio=
_fs_vq *fsvq)
 =09return 0;
 }
=20
+static int notify_complete_waiting_req(struct virtio_fs *vfs,
+=09=09=09=09       struct fuse_notify_lock_out *out_args)
+{
+=09struct virtio_fs_vq *fsvq =3D &vfs->vqs[VQ_REQUEST];
+=09struct fuse_req *req, *next;
+=09bool found =3D false;
+=09struct fuse_conn *fc =3D fsvq->fud->fc;
+
+=09/* Find waiting request with the unique number and end it */
+=09spin_lock(&fsvq->lock);
+=09=09list_for_each_entry_safe(req, next, &fsvq->wait_reqs, list) {
+=09=09=09if (req->in.h.unique =3D=3D out_args->id) {
+=09=09=09=09list_del_init(&req->list);
+=09=09=09=09clear_bit(FR_SENT, &req->flags);
+=09=09=09=09/* Transfer error code from notify */
+=09=09=09=09req->out.h.error =3D out_args->error;
+=09=09=09=09found =3D true;
+=09=09=09=09break;
+=09=09=09}
+=09=09}
+=09spin_unlock(&fsvq->lock);
+
+=09/*
+=09 * TODO: It is possible that some re-ordering happens in notify
+=09 * comes before request is complete. Deal with it.
+=09 */
+=09if (found) {
+=09=09fuse_request_end(fc, req);
+=09=09spin_lock(&fsvq->lock);
+=09=09dec_in_flight_req(fsvq);
+=09=09spin_unlock(&fsvq->lock);
+=09} else
+=09=09pr_debug("virtio-fs: Did not find waiting request with"
+=09=09       " unique=3D0x%llx\n", out_args->id);
+
+=09return 0;
+}
+
+static int virtio_fs_handle_notify(struct virtio_fs *vfs,
+=09=09=09=09   struct virtio_fs_notify *notify)
+{
+=09int ret =3D 0;
+=09struct fuse_out_header *oh =3D &notify->out_hdr;
+=09struct fuse_notify_lock_out *lo;
+
+=09/*
+=09 * For notifications, oh.unique is 0 and oh->error contains code
+=09 * for which notification as arrived.
+=09 */
+=09switch(oh->error) {
+=09case FUSE_NOTIFY_LOCK:
+=09=09lo =3D (struct fuse_notify_lock_out *) &notify->outarg;
+=09=09notify_complete_waiting_req(vfs, lo);
+=09=09break;
+=09default:
+=09=09printk("virtio-fs: Unexpected notification %d\n", oh->error);
+=09}
+=09return ret;
+}
+
 static void virtio_fs_notify_done_work(struct work_struct *work)
 {
 =09struct virtio_fs_vq *fsvq =3D container_of(work, struct virtio_fs_vq,
 =09=09=09=09=09=09 done_work);
 =09struct virtqueue *vq =3D fsvq->vq;
+=09struct virtio_fs *vfs =3D vq->vdev->priv;
 =09LIST_HEAD(reqs);
 =09struct virtio_fs_notify_node *notify, *next;
+=09struct fuse_out_header *oh;
=20
 =09spin_lock(&fsvq->lock);
 =09do {
@@ -535,6 +597,10 @@ static void virtio_fs_notify_done_work(struct work_str=
uct *work)
=20
 =09/* Process notify */
 =09list_for_each_entry_safe(notify, next, &reqs, list) {
+=09=09oh =3D &notify->notify.out_hdr;
+=09=09WARN_ON(oh->unique);
+=09=09/* Handle notification */
+=09=09virtio_fs_handle_notify(vfs, &notify->notify);
 =09=09spin_lock(&fsvq->lock);
 =09=09dec_in_flight_req(fsvq);
 =09=09list_del_init(&notify->list);
@@ -656,6 +722,15 @@ static void virtio_fs_requests_done_work(struct work_s=
truct *work)
 =09=09 * TODO verify that server properly follows FUSE protocol
 =09=09 * (oh.uniq, oh.len)
 =09=09 */
+=09=09if (req->out.h.error =3D=3D 1) {
+=09=09=09/* Wait for notification to complete request */
+=09=09=09list_del_init(&req->list);
+=09=09=09spin_lock(&fsvq->lock);
+=09=09=09list_add_tail(&req->list, &fsvq->wait_reqs);
+=09=09=09spin_unlock(&fsvq->lock);
+=09=09=09continue;
+=09=09}
+
 =09=09args =3D req->args;
 =09=09copy_args_from_argbuf(args, req);
=20
@@ -705,6 +780,7 @@ static int virtio_fs_init_vq(struct virtio_fs *fs, stru=
ct virtio_fs_vq *fsvq,
 =09strncpy(fsvq->name, name, VQ_NAME_LEN);
 =09spin_lock_init(&fsvq->lock);
 =09INIT_LIST_HEAD(&fsvq->queued_reqs);
+=09INIT_LIST_HEAD(&fsvq->wait_reqs);
 =09INIT_LIST_HEAD(&fsvq->end_reqs);
 =09INIT_LIST_HEAD(&fsvq->notify_reqs);
 =09init_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 {
 =09FUSE_NOTIFY_STORE =3D 4,
 =09FUSE_NOTIFY_RETRIEVE =3D 5,
 =09FUSE_NOTIFY_DELETE =3D 6,
+=09FUSE_NOTIFY_LOCK =3D 7,
 =09FUSE_NOTIFY_CODE_MAX,
 };
=20
@@ -868,6 +869,12 @@ struct fuse_notify_retrieve_in {
 =09uint64_t=09dummy4;
 };
=20
+struct fuse_notify_lock_out {
+=09uint64_t=09id;
+=09int32_t=09=09error;
+=09int32_t=09=09padding;
+};
+
 /* Device ioctls: */
 #define FUSE_DEV_IOC_CLONE=09_IOR(229, 0, uint32_t)
=20
--=20
2.20.1