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Commit 51fecdd2 authored by Vivek Goyal's avatar Vivek Goyal Committed by Miklos Szeredi
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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: default avatarVivek Goyal <vgoyal@redhat.com>
Signed-off-by: default avatarMiklos Szeredi <mszeredi@redhat.com>
parent 6c26f717

fs/fuse/virtio_fs.c

+33 −4
Original line number Diff line number Diff line
@@ -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 @@ __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, 0);

		spin_unlock(&fsvq->lock);

		return;

	}

}