Re: [PATCH] fs,block: get holder during claim

Yi Zhang
2024-03-18T09:11:15Z

On Fri, Mar 15, 2024 at 9:23 PM Christian Brauner <brauner@kernel.org> wrote:
>
> Now that we open block devices as files we need to deal with the
> realities that closing is a deferred operation. An operation on the
> block device such as e.g., freeze, thaw, or removal that runs
> concurrently with umount, tries to acquire a stable reference on the
> holder. The holder might already be gone though. Make that reliable by
> grabbing a passive reference to the holder during bdev_open() and
> releasing it during bdev_release().
>
> Fixes: f3a608827d1f ("bdev: open block device as files") # mainline only
> Reported-by: Christoph Hellwig <hch@infradead.org>
> Link: https://lore.kernel.org/r/ZfEQQ9jZZVes0WCZ@infradead.org
> Signed-off-by: Christian Brauner <brauner@kernel.org>

Verified the blktests ndb/003 panic issue was fixed by this patch,
feel free to add:

Tested-by: Yi Zhang <yi.zhang@redhat.com>

> ---
> Hey all,
>
> I ran blktests with nbd enabled which contains a reliable repro for the
> issue. Thanks to Christoph for pointing in that direction. The
> underlying issue is not reproducible anymore with this patch applied.
> xfstests and blktests pass.
>
> Thanks!
> Christian
> ---
> block/bdev.c | 7 +++++++
> fs/super.c | 18 ++++++++++++++++++
> include/linux/blkdev.h | 10 ++++++++++
> 3 files changed, 35 insertions(+)
>
> diff --git a/block/bdev.c b/block/bdev.c
> index e7adaaf1c219..7a5f611c3d2e 100644
> --- a/block/bdev.c
> +++ b/block/bdev.c
> @@ -583,6 +583,9 @@ static void bd_finish_claiming(struct block_device *bdev, void *holder,
> mutex_unlock(&bdev->bd_holder_lock);
> bd_clear_claiming(whole, holder);
> mutex_unlock(&bdev_lock);
> +
> + if (hops && hops->get_holder)
> + hops->get_holder(holder);
> }
>
> /**
> @@ -605,6 +608,7 @@ EXPORT_SYMBOL(bd_abort_claiming);
> static void bd_end_claim(struct block_device *bdev, void *holder)
> {
> struct block_device *whole = bdev_whole(bdev);
> + const struct blk_holder_ops *hops = bdev->bd_holder_ops;
> bool unblock = false;
>
> /*
> @@ -627,6 +631,9 @@ static void bd_end_claim(struct block_device *bdev, void *holder)
> whole->bd_holder = NULL;
> mutex_unlock(&bdev_lock);
>
> + if (hops && hops->put_holder)
> + hops->put_holder(holder);
> +
> /*
> * If this was the last claim, remove holder link and unblock evpoll if
> * it was a write holder.
> diff --git a/fs/super.c b/fs/super.c
> index ee05ab6b37e7..71d9779c42b1 100644
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -1515,11 +1515,29 @@ static int fs_bdev_thaw(struct block_device *bdev)
> return error;
> }
>
> +static void fs_bdev_super_get(void *data)
> +{
> + struct super_block *sb = data;
> +
> + spin_lock(&sb_lock);
> + sb->s_count++;
> + spin_unlock(&sb_lock);
> +}
> +
> +static void fs_bdev_super_put(void *data)
> +{
> + struct super_block *sb = data;
> +
> + put_super(sb);
> +}
> +
> const struct blk_holder_ops fs_holder_ops = {
> .mark_dead = fs_bdev_mark_dead,
> .sync = fs_bdev_sync,
> .freeze = fs_bdev_freeze,
> .thaw = fs_bdev_thaw,
> + .get_holder = fs_bdev_super_get,
> + .put_holder = fs_bdev_super_put,
> };
> EXPORT_SYMBOL_GPL(fs_holder_ops);
>
> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> index f9b87c39cab0..c3e8f7cf96be 100644
> --- a/include/linux/blkdev.h
> +++ b/include/linux/blkdev.h
> @@ -1505,6 +1505,16 @@ struct blk_holder_ops {
> * Thaw the file system mounted on the block device.
> */
> int (*thaw)(struct block_device *bdev);
> +
> + /*
> + * If needed, get a reference to the holder.
> + */
> + void (*get_holder)(void *holder);
> +
> + /*
> + * Release the holder.
> + */
> + void (*put_holder)(void *holder);
> };
>
> /*
> --
> 2.43.0
>
>

--
Best Regards,
Yi Zhang