Re: [PATCH 2/3] ceph: clean up locking annotation for ceph_get_snap_realm and __lookup_snap_realm

[Ilya Dryomov <idryomov@gmail.com>]

On Thu, Jun 3, 2021 at 6:52 PM Jeff Layton <jlayton@kernel.org> wrote:
>
> They both say that the snap_rwsem must be held for write, but I don't
> see any real reason for it, and it's not currently always called that
> way.
>
> The lookup is just walking the rbtree, so holding it for read should be
> fine there. The "get" is bumping the refcount and (possibly) removing
> it from the empty list. I see no need to hold the snap_rwsem for write
> for that.
>
> Signed-off-by: Jeff Layton <jlayton@kernel.org>
> ---
>  fs/ceph/snap.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/fs/ceph/snap.c b/fs/ceph/snap.c
> index bc6c33d485e6..f8cac2abab3f 100644
> --- a/fs/ceph/snap.c
> +++ b/fs/ceph/snap.c
> @@ -60,12 +60,12 @@
>  /*
>   * increase ref count for the realm
>   *
> - * caller must hold snap_rwsem for write.
> + * caller must hold snap_rwsem.
>   */
>  void ceph_get_snap_realm(struct ceph_mds_client *mdsc,
>                          struct ceph_snap_realm *realm)
>  {
> -       lockdep_assert_held_write(&mdsc->snap_rwsem);
> +       lockdep_assert_held(&mdsc->snap_rwsem);
>
>         dout("get_realm %p %d -> %d\n", realm,
>              atomic_read(&realm->nref), atomic_read(&realm->nref)+1);
> @@ -139,7 +139,7 @@ static struct ceph_snap_realm *ceph_create_snap_realm(
>  /*
>   * lookup the realm rooted at @ino.
>   *
> - * caller must hold snap_rwsem for write.
> + * caller must hold snap_rwsem.
>   */
>  static struct ceph_snap_realm *__lookup_snap_realm(struct ceph_mds_client *mdsc,
>                                                    u64 ino)
> @@ -147,7 +147,7 @@ static struct ceph_snap_realm *__lookup_snap_realm(struct ceph_mds_client *mdsc,
>         struct rb_node *n = mdsc->snap_realms.rb_node;
>         struct ceph_snap_realm *r;
>
> -       lockdep_assert_held_write(&mdsc->snap_rwsem);
> +       lockdep_assert_held(&mdsc->snap_rwsem);
>
>         while (n) {
>                 r = rb_entry(n, struct ceph_snap_realm, node);
> --
> 2.31.1
>

Ah, since you are relaxing the requirement some of those lockdep
asserts from the previous patch aren't actually redundant.  This seems
fine to me: lookup definitely doesn't need the write lock and allowing
concurrent gets should be OK.  The write lock made some sense back when
get was an atomic_read followed by atomic_inc but not now.

Reviewed-by: Ilya Dryomov <idryomov@gmail.com>

Thanks,

                Ilya