I believe it's possible that we could end up with racing calls to __ceph_remove_cap for the same cap. If that happens, the cap->ci pointer will be zereoed out and we can hit a NULL pointer dereference. Once we acquire the s_cap_lock, check for the ci pointer being NULL, and just return without doing anything if it is. URL: https://tracker.ceph.com/issues/43272 Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx> --- fs/ceph/caps.c | 21 ++++++++++++++++----- 1 file changed, 16 insertions(+), 5 deletions(-) This is the only scenario that made sense to me in light of Ilya's analysis on the tracker above. I could be off here though -- the locking around this code is horrifically complex, and I could be missing what should guard against this scenario. Thoughts? diff --git a/fs/ceph/caps.c b/fs/ceph/caps.c index 9d09bb53c1ab..7e39ee8eff60 100644 --- a/fs/ceph/caps.c +++ b/fs/ceph/caps.c @@ -1046,11 +1046,22 @@ static void drop_inode_snap_realm(struct ceph_inode_info *ci) void __ceph_remove_cap(struct ceph_cap *cap, bool queue_release) { struct ceph_mds_session *session = cap->session; - struct ceph_inode_info *ci = cap->ci; - struct ceph_mds_client *mdsc = - ceph_sb_to_client(ci->vfs_inode.i_sb)->mdsc; + struct ceph_inode_info *ci; + struct ceph_mds_client *mdsc; int removed = 0; + spin_lock(&session->s_cap_lock); + ci = cap->ci; + if (!ci) { + /* + * Did we race with a competing __ceph_remove_cap call? If + * ci is zeroed out, then just unlock and don't do anything. + * Assume that it's on its way out anyway. + */ + spin_unlock(&session->s_cap_lock); + return; + } + dout("__ceph_remove_cap %p from %p\n", cap, &ci->vfs_inode); /* remove from inode's cap rbtree, and clear auth cap */ @@ -1058,13 +1069,12 @@ void __ceph_remove_cap(struct ceph_cap *cap, bool queue_release) if (ci->i_auth_cap == cap) ci->i_auth_cap = NULL; - /* remove from session list */ - spin_lock(&session->s_cap_lock); if (session->s_cap_iterator == cap) { /* not yet, we are iterating over this very cap */ dout("__ceph_remove_cap delaying %p removal from session %p\n", cap, cap->session); } else { + /* remove from session list */ list_del_init(&cap->session_caps); session->s_nr_caps--; cap->session = NULL; @@ -1072,6 +1082,7 @@ void __ceph_remove_cap(struct ceph_cap *cap, bool queue_release) } /* protect backpointer with s_cap_lock: see iterate_session_caps */ cap->ci = NULL; + mdsc = ceph_sb_to_client(ci->vfs_inode.i_sb)->mdsc; /* * s_cap_reconnect is protected by s_cap_lock. no one changes -- 2.23.0
