On Thu, Dec 12, 2019 at 12:31:59PM -0500, Jeff Layton wrote: > 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? This patch _seems_ to make sense. But, as you said, the locking code is incredibly complex. I tried to understand if __send_cap() could have a similar race by accessing cap->ci without s_cap_lock. But I couldn't reach a conclusion :-/ Cheers, -- Luís > 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 >
