On Wed 26-01-22 16:50:40, Christoph Hellwig wrote:
> From: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
>
> The kernel test robot is reporting that xfstest can fail at
>
> umount ext2 on xfs
> umount xfs
>
> sequence, for commit 322c4293ecc58110 ("loop: make autoclear operation
> asynchronous") broke what commit ("loop: Make explicit loop device
> destruction lazy") wanted to achieve.
>
> Although we cannot guarantee that nobody is holding a reference when
> "umount xfs" is called, we should try to close a race window opened
> by asynchronous autoclear operation.
>
> It turned out that there is no need to call flush_workqueue() from
> __loop_clr_fd(), for blk_mq_freeze_queue() blocks until all pending
> "struct work_struct" are processed by loop_process_work().
>
> Revert commit 322c4293ecc58110 ("loop: make autoclear operation
> asynchronous"), and simply defer calling destroy_workqueue() till
> loop_remove() after ensuring that the worker rbtree and repaing
> timer are kept alive while the loop device exists.
>
> Since a loop device is likely reused after once created thanks to
> ioctl(LOOP_CTL_GET_FREE), being unable to destroy corresponding WQ
> when ioctl(LOOP_CLR_FD) is called should be an acceptable waste.
>
> Reported-by: kernel test robot <oliver.sang@xxxxxxxxx>
> Suggested-by: Christoph Hellwig <hch@xxxxxx>
> Tested-by: syzbot <syzbot+643e4ce4b6ad1347d372@xxxxxxxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
> [hch: rebased, keep the work rbtree and timer around longer]
> Signed-off-by: Christoph Hellwig <hch@xxxxxx>
Looks good to me. Feel free to add:
Reviewed-by: Jan Kara <jack@xxxxxxx>
Honza
> ---
> drivers/block/loop.c | 71 ++++++++++++++------------------------------
> drivers/block/loop.h | 1 -
> 2 files changed, 23 insertions(+), 49 deletions(-)
>
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index fc0cdd1612b1d..cf7830a68113a 100644
> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -1038,10 +1038,10 @@ static int loop_configure(struct loop_device *lo, fmode_t mode,
> !file->f_op->write_iter)
> lo->lo_flags |= LO_FLAGS_READ_ONLY;
>
> - lo->workqueue = alloc_workqueue("loop%d",
> - WQ_UNBOUND | WQ_FREEZABLE,
> - 0,
> - lo->lo_number);
> + if (!lo->workqueue)
> + lo->workqueue = alloc_workqueue("loop%d",
> + WQ_UNBOUND | WQ_FREEZABLE,
> + 0, lo->lo_number);
> if (!lo->workqueue) {
> error = -ENOMEM;
> goto out_unlock;
> @@ -1147,10 +1147,6 @@ static void __loop_clr_fd(struct loop_device *lo, bool release)
> if (!release)
> blk_mq_freeze_queue(lo->lo_queue);
>
> - destroy_workqueue(lo->workqueue);
> - loop_free_idle_workers(lo, true);
> - del_timer_sync(&lo->timer);
> -
> spin_lock_irq(&lo->lo_lock);
> filp = lo->lo_backing_file;
> lo->lo_backing_file = NULL;
> @@ -1176,9 +1172,11 @@ static void __loop_clr_fd(struct loop_device *lo, bool release)
> if (lo->lo_flags & LO_FLAGS_PARTSCAN) {
> int err;
>
> - mutex_lock(&lo->lo_disk->open_mutex);
> + if (!release)
> + mutex_lock(&lo->lo_disk->open_mutex);
> err = bdev_disk_changed(lo->lo_disk, false);
> - mutex_unlock(&lo->lo_disk->open_mutex);
> + if (!release)
> + mutex_unlock(&lo->lo_disk->open_mutex);
> if (err)
> pr_warn("%s: partition scan of loop%d failed (rc=%d)\n",
> __func__, lo->lo_number, err);
> @@ -1189,39 +1187,17 @@ static void __loop_clr_fd(struct loop_device *lo, bool release)
> if (!part_shift)
> lo->lo_disk->flags |= GENHD_FL_NO_PART;
>
> - fput(filp);
> -}
> -
> -static void loop_rundown_completed(struct loop_device *lo)
> -{
> mutex_lock(&lo->lo_mutex);
> lo->lo_state = Lo_unbound;
> mutex_unlock(&lo->lo_mutex);
> module_put(THIS_MODULE);
> -}
> -
> -static void loop_rundown_workfn(struct work_struct *work)
> -{
> - struct loop_device *lo = container_of(work, struct loop_device,
> - rundown_work);
> - struct block_device *bdev = lo->lo_device;
> - struct gendisk *disk = lo->lo_disk;
>
> - __loop_clr_fd(lo, true);
> - kobject_put(&bdev->bd_device.kobj);
> - module_put(disk->fops->owner);
> - loop_rundown_completed(lo);
> -}
> -
> -static void loop_schedule_rundown(struct loop_device *lo)
> -{
> - struct block_device *bdev = lo->lo_device;
> - struct gendisk *disk = lo->lo_disk;
> -
> - __module_get(disk->fops->owner);
> - kobject_get(&bdev->bd_device.kobj);
> - INIT_WORK(&lo->rundown_work, loop_rundown_workfn);
> - queue_work(system_long_wq, &lo->rundown_work);
> + /*
> + * Need not hold lo_mutex to fput backing file. Calling fput holding
> + * lo_mutex triggers a circular lock dependency possibility warning as
> + * fput can take open_mutex which is usually taken before lo_mutex.
> + */
> + fput(filp);
> }
>
> static int loop_clr_fd(struct loop_device *lo)
> @@ -1254,7 +1230,6 @@ static int loop_clr_fd(struct loop_device *lo)
> mutex_unlock(&lo->lo_mutex);
>
> __loop_clr_fd(lo, false);
> - loop_rundown_completed(lo);
> return 0;
> }
>
> @@ -1753,20 +1728,14 @@ static void lo_release(struct gendisk *disk, fmode_t mode)
> return;
>
> mutex_lock(&lo->lo_mutex);
> - if (lo->lo_flags & LO_FLAGS_AUTOCLEAR) {
> - if (lo->lo_state != Lo_bound)
> - goto out_unlock;
> + if (lo->lo_state == Lo_bound &&
> + (lo->lo_flags & LO_FLAGS_AUTOCLEAR)) {
> lo->lo_state = Lo_rundown;
> mutex_unlock(&lo->lo_mutex);
> - /*
> - * In autoclear mode, stop the loop thread
> - * and remove configuration after last close.
> - */
> - loop_schedule_rundown(lo);
> + __loop_clr_fd(lo, true);
> return;
> }
>
> -out_unlock:
> mutex_unlock(&lo->lo_mutex);
> }
>
> @@ -2056,6 +2025,12 @@ static void loop_remove(struct loop_device *lo)
> mutex_lock(&loop_ctl_mutex);
> idr_remove(&loop_index_idr, lo->lo_number);
> mutex_unlock(&loop_ctl_mutex);
> +
> + if (lo->workqueue)
> + destroy_workqueue(lo->workqueue);
> + loop_free_idle_workers(lo, true);
> + del_timer_sync(&lo->timer);
> +
> /* There is no route which can find this loop device. */
> mutex_destroy(&lo->lo_mutex);
> kfree(lo);
> diff --git a/drivers/block/loop.h b/drivers/block/loop.h
> index 918a7a2dc0259..082d4b6bfc6a6 100644
> --- a/drivers/block/loop.h
> +++ b/drivers/block/loop.h
> @@ -56,7 +56,6 @@ struct loop_device {
> struct gendisk *lo_disk;
> struct mutex lo_mutex;
> bool idr_visible;
> - struct work_struct rundown_work;
> };
>
> struct loop_cmd {
> --
> 2.30.2
>
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
