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SubjectRe: [PATCH 16/18] virtiofs: Use virtio_fs_mutex for races w.r.t ->remove and mount path
On Fri, Sep 06, 2019 at 01:05:34PM +0100, Stefan Hajnoczi wrote:
> On Thu, Sep 05, 2019 at 03:48:57PM -0400, Vivek Goyal wrote:
> > It is possible that a mount is in progress and device is being removed at
> > the same time. Use virtio_fs_mutex to avoid races.
> >
> > This also takes care of bunch of races and removes some TODO items.
> >
> > Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
> > ---
> > fs/fuse/virtio_fs.c | 32 ++++++++++++++++++++++----------
> > 1 file changed, 22 insertions(+), 10 deletions(-)
>
> Let's move to a per-device mutex in the future. That way a single
> device that fails to drain/complete requests will not hang all other
> virtio-fs instances. This is fine for now.

Good point. For now I updated the patch so that it applies cleanly
after previous two patches changed.

Subject: virtiofs: Use virtio_fs_mutex for races w.r.t ->remove and mount path

It is possible that a mount is in progress and device is being removed at
the same time. Use virtio_fs_mutex to avoid races.

This also takes care of bunch of races and removes some TODO items.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
---
fs/fuse/virtio_fs.c | 32 ++++++++++++++++++++++----------
1 file changed, 22 insertions(+), 10 deletions(-)

Index: rhvgoyal-linux-fuse/fs/fuse/virtio_fs.c
===================================================================
--- rhvgoyal-linux-fuse.orig/fs/fuse/virtio_fs.c 2019-09-06 09:40:53.309245246 -0400
+++ rhvgoyal-linux-fuse/fs/fuse/virtio_fs.c 2019-09-06 09:43:25.335245246 -0400
@@ -13,7 +13,9 @@
#include <linux/highmem.h>
#include "fuse_i.h"

-/* List of virtio-fs device instances and a lock for the list */
+/* List of virtio-fs device instances and a lock for the list. Also provides
+ * mutual exclusion in device removal and mounting path
+ */
static DEFINE_MUTEX(virtio_fs_mutex);
static LIST_HEAD(virtio_fs_instances);

@@ -72,17 +74,19 @@ static void release_virtio_fs_obj(struct
kfree(vfs);
}

+/* Make sure virtiofs_mutex is held */
static void virtio_fs_put(struct virtio_fs *fs)
{
- mutex_lock(&virtio_fs_mutex);
kref_put(&fs->refcount, release_virtio_fs_obj);
- mutex_unlock(&virtio_fs_mutex);
}

static void virtio_fs_fiq_release(struct fuse_iqueue *fiq)
{
struct virtio_fs *vfs = fiq->priv;
+
+ mutex_lock(&virtio_fs_mutex);
virtio_fs_put(vfs);
+ mutex_unlock(&virtio_fs_mutex);
}

static void virtio_fs_drain_queue(struct virtio_fs_vq *fsvq)
@@ -596,9 +600,8 @@ static void virtio_fs_remove(struct virt
struct virtio_fs *fs = vdev->priv;

mutex_lock(&virtio_fs_mutex);
+ /* This device is going away. No one should get new reference */
list_del_init(&fs->list);
- mutex_unlock(&virtio_fs_mutex);
-
virtio_fs_stop_all_queues(fs);
virtio_fs_drain_all_queues(fs);
vdev->config->reset(vdev);
@@ -607,6 +610,7 @@ static void virtio_fs_remove(struct virt
vdev->priv = NULL;
/* Put device reference on virtio_fs object */
virtio_fs_put(fs);
+ mutex_unlock(&virtio_fs_mutex);
}

#ifdef CONFIG_PM_SLEEP
@@ -978,10 +982,15 @@ static int virtio_fs_fill_super(struct s
.no_force_umount = true,
};

- /* TODO lock */
- if (fs->vqs[VQ_REQUEST].fud) {
- pr_err("virtio-fs: device already in use\n");
- err = -EBUSY;
+ mutex_lock(&virtio_fs_mutex);
+
+ /* After holding mutex, make sure virtiofs device is still there.
+ * Though we are holding a refernce to it, drive ->remove might
+ * still have cleaned up virtual queues. In that case bail out.
+ */
+ err = -EINVAL;
+ if (list_empty(&fs->list)) {
+ pr_info("virtio-fs: tag <%s> not found\n", fs->tag);
goto err;
}

@@ -1007,7 +1016,6 @@ static int virtio_fs_fill_super(struct s

fc = fs->vqs[VQ_REQUEST].fud->fc;

- /* TODO take fuse_mutex around this loop? */
for (i = 0; i < fs->nvqs; i++) {
struct virtio_fs_vq *fsvq = &fs->vqs[i];

@@ -1020,6 +1028,7 @@ static int virtio_fs_fill_super(struct s
/* Previous unmount will stop all queues. Start these again */
virtio_fs_start_all_queues(fs);
fuse_send_init(fc, init_req);
+ mutex_unlock(&virtio_fs_mutex);
return 0;

err_free_init_req:
@@ -1027,6 +1036,7 @@ err_free_init_req:
err_free_fuse_devs:
virtio_fs_free_devs(fs);
err:
+ mutex_unlock(&virtio_fs_mutex);
return err;
}

@@ -1100,7 +1110,9 @@ static int virtio_fs_get_tree(struct fs_

fc = kzalloc(sizeof(struct fuse_conn), GFP_KERNEL);
if (!fc) {
+ mutex_lock(&virtio_fs_mutex);
virtio_fs_put(fs);
+ mutex_unlock(&virtio_fs_mutex);
return -ENOMEM;
}

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