linux-kernel - Re: [PATCH 16/18] virtiofs: Use virtio_fs_mutex for races w.r.t ->remove and mount path

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Date:   Mon, 9 Sep 2019 18:13:05 +0200
From:   Stefan Hajnoczi <stefanha@...hat.com>
To:     Vivek Goyal <vgoyal@...hat.com>
Cc:     linux-fsdevel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, miklos@...redi.hu,
        linux-kernel@...r.kernel.org, virtio-fs@...hat.com,
        dgilbert@...hat.com, mst@...hat.com
Subject: Re: [PATCH 16/18] virtiofs: Use virtio_fs_mutex for races w.r.t
 ->remove and mount path

On Fri, Sep 06, 2019 at 09:51:31AM -0400, Vivek Goyal wrote:
> 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@...hat.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@...hat.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;
>  	}
>  
> 

Reviewed-by: Stefan Hajnoczi <stefanha@...hat.com>

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