virtiofs: Do not end request in submission context

Submission context can hold some locks which end request code tries to hold
again and deadlock can occur. For example, fc->bg_lock. If a background
request is being submitted, it might hold fc->bg_lock and if we could not
submit request (because device went away) and tried to end request, then
deadlock happens. During testing, I also got a warning from deadlock
detection code.

So put requests on a list and end requests from a worker thread.

I got following warning from deadlock detector.

[  603.137138] WARNING: possible recursive locking detected
[  603.137142] --------------------------------------------
[  603.137144] blogbench/2036 is trying to acquire lock:
[  603.137149] 00000000f0f51107 (&(&fc->bg_lock)->rlock){+.+.}, at: fuse_request_end+0xdf/0x1c0 [fuse]
[  603.140701]
[  603.140701] but task is already holding lock:
[  603.140703] 00000000f0f51107 (&(&fc->bg_lock)->rlock){+.+.}, at: fuse_simple_background+0x92/0x1d0 [fuse]
[  603.140713]
[  603.140713] other info that might help us debug this:
[  603.140714]  Possible unsafe locking scenario:
[  603.140714]
[  603.140715]        CPU0
[  603.140716]        ----
[  603.140716]   lock(&(&fc->bg_lock)->rlock);
[  603.140718]   lock(&(&fc->bg_lock)->rlock);
[  603.140719]
[  603.140719]  *** DEADLOCK ***

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

1 files changed, 33 insertions, 4 deletions

diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c
index e22a0c003c3d..7ea58606cc1d 100644
--- a/fs/fuse/virtio_fs.c
+++ b/fs/fuse/virtio_fs.c
@@ -30,6 +30,7 @@ struct virtio_fs_vq {
         struct virtqueue *vq;     /* protected by ->lock */
         struct work_struct done_work;
         struct list_head queued_reqs;
+        struct list_head end_reqs;      /* End these requests */
         struct delayed_work dispatch_work;
         struct fuse_dev *fud;
         bool connected;
@@ -259,8 +260,27 @@ static void virtio_fs_hiprio_done_work(struct work_struct *work)
         spin_unlock(&fsvq->lock);
 }
 
-static void virtio_fs_dummy_dispatch_work(struct work_struct *work)
+static void virtio_fs_request_dispatch_work(struct work_struct *work)
 {
+        struct fuse_req *req;
+        struct virtio_fs_vq *fsvq = container_of(work, struct virtio_fs_vq,
+                                                 dispatch_work.work);
+        struct fuse_conn *fc = fsvq->fud->fc;
+
+        pr_debug("virtio-fs: worker %s called.\n", __func__);
+        while (1) {
+                spin_lock(&fsvq->lock);
+                req = list_first_entry_or_null(&fsvq->end_reqs, struct fuse_req,
+                                               list);
+                if (!req) {
+                        spin_unlock(&fsvq->lock);
+                        return;
+                }
+
+                list_del_init(&req->list);
+                spin_unlock(&fsvq->lock);
+                fuse_request_end(fc, req);
+        }
 }
 
 static void virtio_fs_hiprio_dispatch_work(struct work_struct *work)
@@ -502,6 +522,7 @@ static int virtio_fs_setup_vqs(struct virtio_device *vdev,
         names[VQ_HIPRIO] = fs->vqs[VQ_HIPRIO].name;
         INIT_WORK(&fs->vqs[VQ_HIPRIO].done_work, virtio_fs_hiprio_done_work);
         INIT_LIST_HEAD(&fs->vqs[VQ_HIPRIO].queued_reqs);
+        INIT_LIST_HEAD(&fs->vqs[VQ_HIPRIO].end_reqs);
         INIT_DELAYED_WORK(&fs->vqs[VQ_HIPRIO].dispatch_work,
                         virtio_fs_hiprio_dispatch_work);
         spin_lock_init(&fs->vqs[VQ_HIPRIO].lock);
@@ -511,8 +532,9 @@ static int virtio_fs_setup_vqs(struct virtio_device *vdev,
                 spin_lock_init(&fs->vqs[i].lock);
                 INIT_WORK(&fs->vqs[i].done_work, virtio_fs_requests_done_work);
                 INIT_DELAYED_WORK(&fs->vqs[i].dispatch_work,
-                                        virtio_fs_dummy_dispatch_work);
+                                  virtio_fs_request_dispatch_work);
                 INIT_LIST_HEAD(&fs->vqs[i].queued_reqs);
+                INIT_LIST_HEAD(&fs->vqs[i].end_reqs);
                 snprintf(fs->vqs[i].name, sizeof(fs->vqs[i].name),
                          "requests.%u", i - VQ_REQUEST);
                 callbacks[i] = virtio_fs_vq_done;
@@ -918,6 +940,7 @@ __releases(fiq->lock)
         struct fuse_conn *fc;
         struct fuse_req *req;
         struct fuse_pqueue *fpq;
+        struct virtio_fs_vq *fsvq;
         int ret;
 
         WARN_ON(list_empty(&fiq->pending));
@@ -951,7 +974,8 @@ __releases(fiq->lock)
         smp_mb__after_atomic();
 
 retry:
-        ret = virtio_fs_enqueue_req(&fs->vqs[queue_id], req);
+        fsvq = &fs->vqs[queue_id];
+        ret = virtio_fs_enqueue_req(fsvq, req);
         if (ret < 0) {
                 if (ret == -ENOMEM || ret == -ENOSPC) {
                         /* Virtqueue full. Retry submission */
@@ -965,7 +989,12 @@ retry:
                 clear_bit(FR_SENT, &req->flags);
                 list_del_init(&req->list);
                 spin_unlock(&fpq->lock);
-                fuse_request_end(fc, req);
+
+                /* Can't end request in submission context. Use a worker */
+                spin_lock(&fsvq->lock);
+                list_add_tail(&req->list, &fsvq->end_reqs);
+                schedule_delayed_work(&fsvq->dispatch_work, 0);
+                spin_unlock(&fsvq->lock);
                 return;
         }
 }