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@xxxxxxxxxx>
---
fs/fuse/virtio_fs.c | 38 ++++++++++++++++++++++++++++++++++----
1 file changed, 34 insertions(+), 4 deletions(-)
diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c
index 6af3f131e468..24ac6f8bf3f7 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 +989,13 @@ __releases(fiq->lock)
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,
+ msecs_to_jiffies(1));
+ spin_unlock(&fsvq->lock);
return;
}
}
--
2.20.1
