Linux Virtualization - Oct 2019 - [PATCH 5/5] virtiofs: Retry request submission from worker context

Hi,

We have couple of places which can result in deadlock. This patch series
fixes these.

We can be called with fc->bg_lock (for background requests) while
submitting a request. This leads to two constraints.

- We can't end requests in submitter's context and call
fuse_end_request()
  as it tries to take fc->bg_lock as well. So queue these requests on a
  list and use a worker to end these requests.

- If virtqueue is full, we can wait with fc->bg_lock held for queue to
  have space. Worker which is completing the request gets blocked on
  fc->bg_lock as well. And that means requests are not completing, that
  means descriptors are not being freed and that means submitter can't
  make progress. Deadlock. 

  Fix this by punting the requests to a list and retry submission later
  with the help of a worker.

Thanks
Vivek

Vivek Goyal (5):
  virtiofs: Do not end request in submission context
  virtiofs: No need to check fpq->connected state
  virtiofs: Set FR_SENT flag only after request has been sent
  virtiofs: Count pending forgets as in_flight forgets
  virtiofs: Retry request submission from worker context

 fs/fuse/virtio_fs.c | 165 +++++++++++++++++++++++++++++---------------
 1 file changed, 111 insertions(+), 54 deletions(-)

-- 
2.20.1

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 at redhat.com>
---
 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

In virtiofs we keep per queue connected state in virtio_fs_vq->connected
and use that to end request if queue is not connected. And virtiofs does
not even touch fpq->connected state.

We probably need to merge these two at some point of time. For now, simplify
the code a bit and do not worry about checking state of fpq->connected.

Signed-off-by: Vivek Goyal <vgoyal at redhat.com>
---
 fs/fuse/virtio_fs.c | 7 -------
 1 file changed, 7 deletions(-)

diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c
index 24ac6f8bf3f7..3b7f7409e77b 100644
--- a/fs/fuse/virtio_fs.c
+++ b/fs/fuse/virtio_fs.c
@@ -960,13 +960,6 @@ __releases(fiq->lock)
 
 	fpq = &fs->vqs[queue_id].fud->pq;
 	spin_lock(&fpq->lock);
-	if (!fpq->connected) {
-		spin_unlock(&fpq->lock);
-		req->out.h.error = -ENODEV;
-		pr_err("virtio-fs: %s disconnected\n", __func__);
-		fuse_request_end(fc, req);
-		return;
-	}
 	list_add_tail(&req->list, fpq->processing);
 	spin_unlock(&fpq->lock);
 	set_bit(FR_SENT, &req->flags);
-- 
2.20.1

FR_SENT flag should be set when request has been sent successfuly sent
over virtqueue. This is used by interrupt logic to figure out if interrupt
request should be sent or not.

Also add it to fqp->processing list after sending it successfully.

Signed-off-by: Vivek Goyal <vgoyal at redhat.com>
---
 fs/fuse/virtio_fs.c | 23 ++++++++++-------------
 1 file changed, 10 insertions(+), 13 deletions(-)

diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c
index 3b7f7409e77b..e0fcf3030951 100644
--- a/fs/fuse/virtio_fs.c
+++ b/fs/fuse/virtio_fs.c
@@ -857,6 +857,7 @@ static int virtio_fs_enqueue_req(struct virtio_fs_vq *fsvq,
 	unsigned int i;
 	int ret;
 	bool notify;
+	struct fuse_pqueue *fpq;
 
 	/* Does the sglist fit on the stack? */
 	total_sgs = sg_count_fuse_req(req);
@@ -911,6 +912,15 @@ static int virtio_fs_enqueue_req(struct virtio_fs_vq *fsvq,
 		goto out;
 	}
 
+	/* Request successfuly sent. */
+	fpq = &fsvq->fud->pq;
+	spin_lock(&fpq->lock);
+	list_add_tail(&req->list, fpq->processing);
+	spin_unlock(&fpq->lock);
+	set_bit(FR_SENT, &req->flags);
+	/* matches barrier in request_wait_answer() */
+	smp_mb__after_atomic();
+
 	fsvq->in_flight++;
 	notify = virtqueue_kick_prepare(vq);
 
@@ -939,7 +949,6 @@ __releases(fiq->lock)
 	struct virtio_fs *fs;
 	struct fuse_conn *fc;
 	struct fuse_req *req;
-	struct fuse_pqueue *fpq;
 	struct virtio_fs_vq *fsvq;
 	int ret;
 
@@ -958,14 +967,6 @@ __releases(fiq->lock)
 		 req->in.h.nodeid, req->in.h.len,
 		 fuse_len_args(req->args->out_numargs, req->args->out_args));
 
-	fpq = &fs->vqs[queue_id].fud->pq;
-	spin_lock(&fpq->lock);
-	list_add_tail(&req->list, fpq->processing);
-	spin_unlock(&fpq->lock);
-	set_bit(FR_SENT, &req->flags);
-	/* matches barrier in request_wait_answer() */
-	smp_mb__after_atomic();
-
 retry:
 	fsvq = &fs->vqs[queue_id];
 	ret = virtio_fs_enqueue_req(fsvq, req);
@@ -978,10 +979,6 @@ __releases(fiq->lock)
 		}
 		req->out.h.error = ret;
 		pr_err("virtio-fs: virtio_fs_enqueue_req() failed %d\n", ret);
-		spin_lock(&fpq->lock);
-		clear_bit(FR_SENT, &req->flags);
-		list_del_init(&req->list);
-		spin_unlock(&fpq->lock);
 
 		/* Can't end request in submission context. Use a worker */
 		spin_lock(&fsvq->lock);
-- 
2.20.1

If virtqueue is full, we put forget requests on a list and these forgets
are dispatched later using a worker. As of now we don't count these
forgets in fsvq->in_flight variable. This means when queue is being drained,
we have to have special logic to first drain these pending requests and
then wait for fsvq->in_flight to go to zero.

By counting pending forgets in fsvq->in_flight, we can get rid of special
logic and just wait for in_flight to go to zero. Worker thread will
kick and drain all the forgets anyway, leading in_flight to zero.

I also need similar logic for normal request queue in next patch where
I am about to defer request submission in the worker context if queue is
full.

This simplifies the code a bit.

Also add two helper functions to inc/dec in_flight. Decrement in_flight
helper will later used to call completion when in_flight reaches zero.

Signed-off-by: Vivek Goyal <vgoyal at redhat.com>
---
 fs/fuse/virtio_fs.c | 44 ++++++++++++++++++++------------------------
 1 file changed, 20 insertions(+), 24 deletions(-)

diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c
index e0fcf3030951..625de45fa471 100644
--- a/fs/fuse/virtio_fs.c
+++ b/fs/fuse/virtio_fs.c
@@ -67,6 +67,19 @@ static inline struct fuse_pqueue *vq_to_fpq(struct virtqueue
*vq)
 	return &vq_to_fsvq(vq)->fud->pq;
 }
 
+/* Should be called with fsvq->lock held. */
+static inline void inc_in_flight_req(struct virtio_fs_vq *fsvq)
+{
+	fsvq->in_flight++;
+}
+
+/* Should be called with fsvq->lock held. */
+static inline void dec_in_flight_req(struct virtio_fs_vq *fsvq)
+{
+	WARN_ON(fsvq->in_flight <= 0);
+	fsvq->in_flight--;
+}
+
 static void release_virtio_fs_obj(struct kref *ref)
 {
 	struct virtio_fs *vfs = container_of(ref, struct virtio_fs, refcount);
@@ -110,22 +123,6 @@ static void virtio_fs_drain_queue(struct virtio_fs_vq
*fsvq)
 	flush_delayed_work(&fsvq->dispatch_work);
 }
 
-static inline void drain_hiprio_queued_reqs(struct virtio_fs_vq *fsvq)
-{
-	struct virtio_fs_forget *forget;
-
-	spin_lock(&fsvq->lock);
-	while (1) {
-		forget = list_first_entry_or_null(&fsvq->queued_reqs,
-						struct virtio_fs_forget, list);
-		if (!forget)
-			break;
-		list_del(&forget->list);
-		kfree(forget);
-	}
-	spin_unlock(&fsvq->lock);
-}
-
 static void virtio_fs_drain_all_queues(struct virtio_fs *fs)
 {
 	struct virtio_fs_vq *fsvq;
@@ -133,9 +130,6 @@ static void virtio_fs_drain_all_queues(struct virtio_fs *fs)
 
 	for (i = 0; i < fs->nvqs; i++) {
 		fsvq = &fs->vqs[i];
-		if (i == VQ_HIPRIO)
-			drain_hiprio_queued_reqs(fsvq);
-
 		virtio_fs_drain_queue(fsvq);
 	}
 }
@@ -254,7 +248,7 @@ static void virtio_fs_hiprio_done_work(struct work_struct
*work)
 
 		while ((req = virtqueue_get_buf(vq, &len)) != NULL) {
 			kfree(req);
-			fsvq->in_flight--;
+			dec_in_flight_req(fsvq);
 		}
 	} while (!virtqueue_enable_cb(vq) &&
likely(!virtqueue_is_broken(vq)));
 	spin_unlock(&fsvq->lock);
@@ -306,6 +300,7 @@ static void virtio_fs_hiprio_dispatch_work(struct
work_struct *work)
 
 		list_del(&forget->list);
 		if (!fsvq->connected) {
+			dec_in_flight_req(fsvq);
 			spin_unlock(&fsvq->lock);
 			kfree(forget);
 			continue;
@@ -327,13 +322,13 @@ static void virtio_fs_hiprio_dispatch_work(struct
work_struct *work)
 			} else {
 				pr_debug("virtio-fs: Could not queue FORGET: err=%d. Dropping
it.\n",
 					 ret);
+				dec_in_flight_req(fsvq);
 				kfree(forget);
 			}
 			spin_unlock(&fsvq->lock);
 			return;
 		}
 
-		fsvq->in_flight++;
 		notify = virtqueue_kick_prepare(vq);
 		spin_unlock(&fsvq->lock);
 
@@ -472,7 +467,7 @@ static void virtio_fs_requests_done_work(struct work_struct
*work)
 
 		fuse_request_end(fc, req);
 		spin_lock(&fsvq->lock);
-		fsvq->in_flight--;
+		dec_in_flight_req(fsvq);
 		spin_unlock(&fsvq->lock);
 	}
 }
@@ -730,6 +725,7 @@ __releases(fiq->lock)
 			list_add_tail(&forget->list, &fsvq->queued_reqs);
 			schedule_delayed_work(&fsvq->dispatch_work,
 					msecs_to_jiffies(1));
+			inc_in_flight_req(fsvq);
 		} else {
 			pr_debug("virtio-fs: Could not queue FORGET: err=%d. Dropping
it.\n",
 				 ret);
@@ -739,7 +735,7 @@ __releases(fiq->lock)
 		goto out;
 	}
 
-	fsvq->in_flight++;
+	inc_in_flight_req(fsvq);
 	notify = virtqueue_kick_prepare(vq);
 
 	spin_unlock(&fsvq->lock);
@@ -921,7 +917,7 @@ static int virtio_fs_enqueue_req(struct virtio_fs_vq *fsvq,
 	/* matches barrier in request_wait_answer() */
 	smp_mb__after_atomic();
 
-	fsvq->in_flight++;
+	inc_in_flight_req(fsvq);
 	notify = virtqueue_kick_prepare(vq);
 
 	spin_unlock(&fsvq->lock);
-- 
2.20.1

If regular request queue gets full, currently we sleep for a bit and
retrying submission in submitter's context. This assumes submitter is
not holding any spin lock. But this assumption is not true for background
requests. For background requests, we are called with fc->bg_lock held.

This can lead to deadlock where one thread is trying submission with
fc->bg_lock held while request completion thread has called
fuse_request_end()
which tries to acquire fc->bg_lock and gets blocked. As request completion
thread gets blocked, it does not make further progress and that means queue
does not get empty and submitter can't submit more requests.

To solve this issue, retry submission with the help of a worker, instead of
retrying in submitter's context. We already do this for hiprio/forget
requests.

Reported-by: Chirantan Ekbote <chirantan at chromium.org>
Signed-off-by: Vivek Goyal <vgoyal at redhat.com>
---
 fs/fuse/virtio_fs.c | 59 ++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 50 insertions(+), 9 deletions(-)

diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c
index 625de45fa471..58e568ef54ef 100644
--- a/fs/fuse/virtio_fs.c
+++ b/fs/fuse/virtio_fs.c
@@ -55,6 +55,9 @@ struct virtio_fs_forget {
 	struct list_head list;
 };
 
+static int virtio_fs_enqueue_req(struct virtio_fs_vq *fsvq,
+				 struct fuse_req *req, bool in_flight);
+
 static inline struct virtio_fs_vq *vq_to_fsvq(struct virtqueue *vq)
 {
 	struct virtio_fs *fs = vq->vdev->priv;
@@ -260,6 +263,7 @@ static void virtio_fs_request_dispatch_work(struct
work_struct *work)
 	struct virtio_fs_vq *fsvq = container_of(work, struct virtio_fs_vq,
 						 dispatch_work.work);
 	struct fuse_conn *fc = fsvq->fud->fc;
+	int ret;
 
 	pr_debug("virtio-fs: worker %s called.\n", __func__);
 	while (1) {
@@ -268,13 +272,43 @@ static void virtio_fs_request_dispatch_work(struct
work_struct *work)
 					       list);
 		if (!req) {
 			spin_unlock(&fsvq->lock);
-			return;
+			break;
 		}
 
 		list_del_init(&req->list);
 		spin_unlock(&fsvq->lock);
 		fuse_request_end(fc, req);
 	}
+
+	/* Dispatch pending requests */
+	while (1) {
+		spin_lock(&fsvq->lock);
+		req = list_first_entry_or_null(&fsvq->queued_reqs,
+					       struct fuse_req, list);
+		if (!req) {
+			spin_unlock(&fsvq->lock);
+			return;
+		}
+		list_del_init(&req->list);
+		spin_unlock(&fsvq->lock);
+
+		ret = virtio_fs_enqueue_req(fsvq, req, true);
+		if (ret < 0) {
+			if (ret == -ENOMEM || ret == -ENOSPC) {
+				spin_lock(&fsvq->lock);
+				list_add_tail(&req->list, &fsvq->queued_reqs);
+				schedule_delayed_work(&fsvq->dispatch_work,
+						      msecs_to_jiffies(1));
+				spin_unlock(&fsvq->lock);
+				return;
+			}
+			req->out.h.error = ret;
+			dec_in_flight_req(fsvq);
+			pr_err("virtio-fs: virtio_fs_enqueue_req() failed %d\n",
+			       ret);
+			fuse_request_end(fc, req);
+		}
+	}
 }
 
 static void virtio_fs_hiprio_dispatch_work(struct work_struct *work)
@@ -837,7 +871,7 @@ static unsigned int sg_init_fuse_args(struct scatterlist
*sg,
 
 /* Add a request to a virtqueue and kick the device */
 static int virtio_fs_enqueue_req(struct virtio_fs_vq *fsvq,
-				 struct fuse_req *req)
+				 struct fuse_req *req, bool in_flight)
 {
 	/* requests need at least 4 elements */
 	struct scatterlist *stack_sgs[6];
@@ -917,7 +951,8 @@ static int virtio_fs_enqueue_req(struct virtio_fs_vq *fsvq,
 	/* matches barrier in request_wait_answer() */
 	smp_mb__after_atomic();
 
-	inc_in_flight_req(fsvq);
+	if (!in_flight)
+		inc_in_flight_req(fsvq);
 	notify = virtqueue_kick_prepare(vq);
 
 	spin_unlock(&fsvq->lock);
@@ -963,15 +998,21 @@ __releases(fiq->lock)
 		 req->in.h.nodeid, req->in.h.len,
 		 fuse_len_args(req->args->out_numargs, req->args->out_args));
 
-retry:
 	fsvq = &fs->vqs[queue_id];
-	ret = virtio_fs_enqueue_req(fsvq, req);
+	ret = virtio_fs_enqueue_req(fsvq, req, false);
 	if (ret < 0) {
 		if (ret == -ENOMEM || ret == -ENOSPC) {
-			/* Virtqueue full. Retry submission */
-			/* TODO use completion instead of timeout */
-			usleep_range(20, 30);
-			goto retry;
+			/*
+			 * Virtqueue full. Retry submission from worker
+			 * context as we might be holding fc->bg_lock.
+			 */
+			spin_lock(&fsvq->lock);
+			list_add_tail(&req->list, &fsvq->queued_reqs);
+			inc_in_flight_req(fsvq);
+			schedule_delayed_work(&fsvq->dispatch_work,
+						msecs_to_jiffies(1));
+			spin_unlock(&fsvq->lock);
+			return;
 		}
 		req->out.h.error = ret;
 		pr_err("virtio-fs: virtio_fs_enqueue_req() failed %d\n", ret);
-- 
2.20.1

On Mon, Oct 21, 2019 at 10:03:39AM +0200, Miklos Szeredi wrote:

[..]
> >  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));
> 
> What's the reason to delay by one msec?  If this is purely for
> deadlock avoidance, then a zero delay would work better, no?
Hi Miklos,

I have no good reason to do that. Will change it to zero delay.

Thanks
Vivek

On Mon, Oct 21, 2019 at 10:15:18AM +0200, Miklos Szeredi wrote:
[..]
> > @@ -268,13 +272,43 @@ static void
virtio_fs_request_dispatch_work(struct work_struct *work)
> >                                                list);
> >                 if (!req) {
> >                         spin_unlock(&fsvq->lock);
> > -                       return;
> > +                       break;
> >                 }
> >
> >                 list_del_init(&req->list);
> >                 spin_unlock(&fsvq->lock);
> >                 fuse_request_end(fc, req);
> >         }
> > +
> > +       /* Dispatch pending requests */
> > +       while (1) {
> > +               spin_lock(&fsvq->lock);
> > +               req =
list_first_entry_or_null(&fsvq->queued_reqs,
> > +                                              struct fuse_req, list);
> > +               if (!req) {
> > +                       spin_unlock(&fsvq->lock);
> > +                       return;
> > +               }
> > +               list_del_init(&req->list);
> > +               spin_unlock(&fsvq->lock);
> > +
> > +               ret = virtio_fs_enqueue_req(fsvq, req, true);
> > +               if (ret < 0) {
> > +                       if (ret == -ENOMEM || ret == -ENOSPC) {
> > +                               spin_lock(&fsvq->lock);
> > +                               list_add_tail(&req->list,
&fsvq->queued_reqs);
> > +                              
schedule_delayed_work(&fsvq->dispatch_work,
> > +                                                    
msecs_to_jiffies(1));
> > +                               spin_unlock(&fsvq->lock);
> > +                               return;
> > +                       }
> > +                       req->out.h.error = ret;
> > +                       dec_in_flight_req(fsvq);
> 
> Missing locking.  Fixed.
Good catch. Thanks.

Vivek