Re: [PATCH 2/7] block: loop: don't hold lo_ctl_mutex in lo_open

[Ming Lei]

Hi Jarod,

On Wed, Apr 8, 2015 at 2:23 PM, Jarod Wilson <jarod@xxxxxxxxxx> wrote:
> From: Ming Lei <ming.lei@xxxxxxxxxxxxx>
>
> The lo_ctl_mutex is held for running all ioctl handlers, and
> in some ioctl handlers, ioctl_by_bdev(BLKRRPART) is called for
> rereading partitions, which requires bd_mutex.
>
> So it is easy to cause failure because trylock(bd_mutex) may
> fail inside blkdev_reread_part(), and follows the lock context:
>
> blkid or other application:
>         ->open()
>                 ->mutex_lock(bd_mutex)
>                 ->lo_open()
>                         ->mutex_lock(lo_ctl_mutex)
>
> losetup(set fd ioctl):
>         ->mutex_lock(lo_ctl_mutex)
>         ->ioctl_by_bdev(BLKRRPART)
>                 ->trylock(bd_mutex)
>
> This patch trys to eliminate the ABBA lock dependency by removing
> lo_ctl_mutext in lo_open() with the following approach:
>
> 1) introduce lo_open_mutex to protect lo_refcnt and avoid acquiring
> lo_ctl_mutex in lo_open():

It is a bit quick since I said the lo_open_mutex can be removed,
and Christoph agreed that too.

So looks we still need to post another version, :-)

>         - for open vs. add/del loop, no any problem because of loop_index_mutex
>         - lo_open_mutex is used for syncing open() and loop_clr_fd()
>         - both open() and release() have been serialized by bd_mutex already
>
> 2) don't hold lo_ctl_mutex for decreasing/checking lo_refcnt in
> lo_release(), then lo_ctl_mutex is only required for the last release.
>
> CC: Christoph Hellwig <hch@xxxxxxxxxxxxx>
> CC: Jens Axboe <axboe@xxxxxxxxx>
> CC: Tejun Heo <tj@xxxxxxxxxx>
> CC: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
> CC: Markus Pargmann <mpa@xxxxxxxxxxxxxx>
> CC: Stefan Weinhuber <wein@xxxxxxxxxx>
> CC: Stefan Haberland <stefan.haberland@xxxxxxxxxx>
> CC: Sebastian Ott <sebott@xxxxxxxxxxxxxxxxxx>
> CC: Fabian Frederick <fabf@xxxxxxxxx>
> CC: Ming Lei <ming.lei@xxxxxxxxxxxxx>
> CC: David Herrmann <dh.herrmann@xxxxxxxxx>
> CC: Mike Galbraith <bitbucket@xxxxxxxxx>
> CC: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> CC: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> CC: nbd-general@xxxxxxxxxxxxxxxxxxxxx
> CC: linux-s390@xxxxxxxxxxxxxxx
> Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxxxxx>
> Signed-off-by: Jarod Wilson <jarod@xxxxxxxxxx>
> ---
>  drivers/block/loop.c | 32 ++++++++++++++++++++++++++------
>  drivers/block/loop.h |  1 +
>  2 files changed, 27 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index d1f168b..81a6bc1 100644
> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -879,14 +879,18 @@ static int loop_clr_fd(struct loop_device *lo)
>          * <dev>/do something like mkfs/losetup -d <dev> causing the losetup -d
>          * command to fail with EBUSY.
>          */
> +       mutex_lock(&lo->lo_open_mutex);
>         if (lo->lo_refcnt > 1) {
> +               mutex_unlock(&lo->lo_open_mutex);
>                 lo->lo_flags |= LO_FLAGS_AUTOCLEAR;
>                 mutex_unlock(&lo->lo_ctl_mutex);
>                 return 0;
>         }
>
> -       if (filp == NULL)
> +       if (filp == NULL) {
> +               mutex_unlock(&lo->lo_open_mutex);
>                 return -EINVAL;
> +       }
>
>         spin_lock_irq(&lo->lo_lock);
>         lo->lo_state = Lo_rundown;
> @@ -919,6 +923,15 @@ static int loop_clr_fd(struct loop_device *lo)
>         lo->lo_state = Lo_unbound;
>         /* This is safe: open() is still holding a reference. */
>         module_put(THIS_MODULE);
> +
> +       /*
> +        * Unlock open_mutex for avoiding -EBUSY of rereading part:
> +        * - try to acquire bd_mutex from reread part
> +        * - another task is opening the loop with holding bd_mutex
> +        *   and trys to acquire open_mutex
> +        */
> +       mutex_unlock(&lo->lo_open_mutex);
> +
>         if (lo->lo_flags & LO_FLAGS_PARTSCAN && bdev)
>                 ioctl_by_bdev(bdev, BLKRRPART, 0);
>         lo->lo_flags = 0;
> @@ -1376,9 +1389,9 @@ static int lo_open(struct block_device *bdev, fmode_t mode)
>                 goto out;
>         }
>
> -       mutex_lock(&lo->lo_ctl_mutex);
> +       mutex_lock(&lo->lo_open_mutex);
>         lo->lo_refcnt++;
> -       mutex_unlock(&lo->lo_ctl_mutex);
> +       mutex_unlock(&lo->lo_open_mutex);
>  out:
>         mutex_unlock(&loop_index_mutex);
>         return err;
> @@ -1387,13 +1400,16 @@ out:
>  static void lo_release(struct gendisk *disk, fmode_t mode)
>  {
>         struct loop_device *lo = disk->private_data;
> -       int err;
> +       int err, ref;
>
> -       mutex_lock(&lo->lo_ctl_mutex);
> +       mutex_lock(&lo->lo_open_mutex);
> +       ref = --lo->lo_refcnt;
> +       mutex_unlock(&lo->lo_open_mutex);
>
> -       if (--lo->lo_refcnt)
> +       if (ref)
>                 goto out;
>
> +       mutex_lock(&lo->lo_ctl_mutex);
>         if (lo->lo_flags & LO_FLAGS_AUTOCLEAR) {
>                 /*
>                  * In autoclear mode, stop the loop thread
> @@ -1646,6 +1662,7 @@ static int loop_add(struct loop_device **l, int i)
>                 disk->flags |= GENHD_FL_NO_PART_SCAN;
>         disk->flags |= GENHD_FL_EXT_DEVT;
>         mutex_init(&lo->lo_ctl_mutex);
> +       mutex_init(&lo->lo_open_mutex);
>         lo->lo_number           = i;
>         spin_lock_init(&lo->lo_lock);
>         disk->major             = LOOP_MAJOR;
> @@ -1763,11 +1780,14 @@ static long loop_control_ioctl(struct file *file, unsigned int cmd,
>                         mutex_unlock(&lo->lo_ctl_mutex);
>                         break;
>                 }
> +               mutex_lock(&lo->lo_open_mutex);
>                 if (lo->lo_refcnt > 0) {
>                         ret = -EBUSY;
> +                       mutex_unlock(&lo->lo_open_mutex);
>                         mutex_unlock(&lo->lo_ctl_mutex);
>                         break;
>                 }
> +               mutex_unlock(&lo->lo_open_mutex);
>                 lo->lo_disk->private_data = NULL;
>                 mutex_unlock(&lo->lo_ctl_mutex);
>                 idr_remove(&loop_index_idr, lo->lo_number);
> diff --git a/drivers/block/loop.h b/drivers/block/loop.h
> index 301c27f..1b4acf2 100644
> --- a/drivers/block/loop.h
> +++ b/drivers/block/loop.h
> @@ -59,6 +59,7 @@ struct loop_device {
>         bool                    write_started;
>         int                     lo_state;
>         struct mutex            lo_ctl_mutex;
> +       struct mutex            lo_open_mutex;
>
>         struct request_queue    *lo_queue;
>         struct blk_mq_tag_set   tag_set;
> --
> 1.8.3.1
>
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