Re: [PATCH RFC] btrfs: don't hold dev_replace rwsem over whole of btrfs_map_block

[Filipe Manana <fdmanana@kernel.org>]

On Fri, May 3, 2024 at 6:35 AM Johannes Thumshirn <jth@kernel.org> wrote:
>
> From: Johannes Thumshirn <johannes.thumshirn@wdc.com>
>
> Don't hold the dev_replace rwsem for the entirety of btrfs_map_block().
>
> It is only needed to protect
> a) calls to find_live_mirror() and
> b) calling into handle_ops_on_dev_replace().
>
> But there is no need to hold the rwsem for any kind of set_io_stripe()
> calls.
>
> So relax taking the dev_replace rwsem to only protect both cases and check
> if the device replace status has changed in the meantime, for which we have
> to re-do the find_live_mirror() calls.
>
> This fixes a deadlock on raid-stripe-tree where device replace performs a
> scrub operation, which in turn calls into btrfs_map_block() to find the
> physical location of the block.

Do you have a stack trace you can paste to the changelog?
That helps make it more clear and greppable.

>
> Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
> ---
>  fs/btrfs/volumes.c | 30 +++++++++++++++++++-----------
>  1 file changed, 19 insertions(+), 11 deletions(-)
>
> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> index a3dc88e420d1..3a842b9960b2 100644
> --- a/fs/btrfs/volumes.c
> +++ b/fs/btrfs/volumes.c
> @@ -6649,14 +6649,9 @@ int btrfs_map_block(struct btrfs_fs_info *fs_info, enum btrfs_map_op op,
>         max_len = btrfs_max_io_len(map, map_offset, &io_geom);
>         *length = min_t(u64, map->chunk_len - map_offset, max_len);
>
> +again:
>         down_read(&dev_replace->rwsem);
>         dev_replace_is_ongoing = btrfs_dev_replace_is_ongoing(dev_replace);
> -       /*
> -        * Hold the semaphore for read during the whole operation, write is
> -        * requested at commit time but must wait.
> -        */
> -       if (!dev_replace_is_ongoing)
> -               up_read(&dev_replace->rwsem);
>
>         switch (map->type & BTRFS_BLOCK_GROUP_PROFILE_MASK) {
>         case BTRFS_BLOCK_GROUP_RAID0:
> @@ -6689,6 +6684,9 @@ int btrfs_map_block(struct btrfs_fs_info *fs_info, enum btrfs_map_op op,
>                 map_blocks_single(map, &io_geom);
>                 break;
>         }
> +
> +       up_read(&dev_replace->rwsem);
> +
>         if (io_geom.stripe_index >= map->num_stripes) {
>                 btrfs_crit(fs_info,
>                            "stripe index math went horribly wrong, got stripe_index=%u, num_stripes=%u",
> @@ -6784,10 +6782,25 @@ int btrfs_map_block(struct btrfs_fs_info *fs_info, enum btrfs_map_op op,
>         if (op != BTRFS_MAP_READ)
>                 io_geom.max_errors = btrfs_chunk_max_errors(map);
>
> +       /*
> +        * Check if something changed the dev_replace state since
> +        * we've checked it for the last time and if redo the whole
> +        * mapping operation.
> +        */
> +       down_read(&dev_replace->rwsem);
> +       if (!dev_replace_is_ongoing &&
> +           btrfs_dev_replace_is_ongoing(dev_replace)) {
> +               up_read(&dev_replace->rwsem);
> +               goto again;

What about the case where we found device replace was running but it's
not running anymore?
I would change the if condition to:  if (dev_replace_is_ongoing !=
btrfs_dev_replace_is_ongoing(dev_replace))

> +       }
> +       up_read(&dev_replace->rwsem);
> +
>         if (dev_replace_is_ongoing && dev_replace->tgtdev != NULL &&

So here we also need to be under the protection of the rwsem before
checking ->tgtdev.
The device replace might finish just after the check, and then we call
handle_ops_on_dev_replace() and use a NULL tgtdev which is not
expected.

Why not remove the up_read() right above, and...

>             op != BTRFS_MAP_READ) {
> +               down_read(&dev_replace->rwsem);
>                 handle_ops_on_dev_replace(op, bioc, dev_replace, logical,
>                                           &io_geom.num_stripes, &io_geom.max_errors);
> +               up_read(&dev_replace->rwsem);

Stop doing here the down_read() + up_read().

>         }

And then do the up_read() here.

Thanks.

>
>         *bioc_ret = bioc;
> @@ -6796,11 +6809,6 @@ int btrfs_map_block(struct btrfs_fs_info *fs_info, enum btrfs_map_op op,
>         bioc->mirror_num = io_geom.mirror_num;
>
>  out:
> -       if (dev_replace_is_ongoing) {
> -               lockdep_assert_held(&dev_replace->rwsem);
> -               /* Unlock and let waiting writers proceed */
> -               up_read(&dev_replace->rwsem);
> -       }
>         btrfs_free_chunk_map(map);
>         return ret;
>  }
> --
> 2.35.3
>
>