ceph: fix cap removal races
The iterate_session_caps helper traverses the session caps list and tries
to grab an inode reference. However, the __ceph_remove_cap was clearing
the inode backpointer _before_ removing itself from the session list,
causing a null pointer dereference.
Clear cap->ci under protection of s_cap_lock to avoid the race, and to
tightly couple the list and backpointer state. Use a local flag to
indicate whether we are releasing the cap, as cap->session may be modified
by a racing thread in iterate_session_caps.
Signed-off-by: Sage Weil <sage@newdream.net>
diff --git a/fs/ceph/caps.c b/fs/ceph/caps.c
index 0c16818..d940053 100644
--- a/fs/ceph/caps.c
+++ b/fs/ceph/caps.c
@@ -858,6 +858,8 @@
}
/*
+ * Remove a cap. Take steps to deal with a racing iterate_session_caps.
+ *
* caller should hold i_lock.
* caller will not hold session s_mutex if called from destroy_inode.
*/
@@ -866,15 +868,10 @@
struct ceph_mds_session *session = cap->session;
struct ceph_inode_info *ci = cap->ci;
struct ceph_mds_client *mdsc = &ceph_client(ci->vfs_inode.i_sb)->mdsc;
+ int removed = 0;
dout("__ceph_remove_cap %p from %p\n", cap, &ci->vfs_inode);
- /* remove from inode list */
- rb_erase(&cap->ci_node, &ci->i_caps);
- cap->ci = NULL;
- 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) {
@@ -885,10 +882,18 @@
list_del_init(&cap->session_caps);
session->s_nr_caps--;
cap->session = NULL;
+ removed = 1;
}
+ /* protect backpointer with s_cap_lock: see iterate_session_caps */
+ cap->ci = NULL;
spin_unlock(&session->s_cap_lock);
- if (cap->session == NULL)
+ /* remove from inode list */
+ rb_erase(&cap->ci_node, &ci->i_caps);
+ if (ci->i_auth_cap == cap)
+ ci->i_auth_cap = NULL;
+
+ if (removed)
ceph_put_cap(cap);
if (!__ceph_is_any_caps(ci) && ci->i_snap_realm) {
diff --git a/fs/ceph/mds_client.c b/fs/ceph/mds_client.c
index eccc0ec..24561a5 100644
--- a/fs/ceph/mds_client.c
+++ b/fs/ceph/mds_client.c
@@ -736,9 +736,10 @@
}
/*
- * Helper to safely iterate over all caps associated with a session.
+ * Helper to safely iterate over all caps associated with a session, with
+ * special care taken to handle a racing __ceph_remove_cap().
*
- * caller must hold session s_mutex
+ * Caller must hold session s_mutex.
*/
static int iterate_session_caps(struct ceph_mds_session *session,
int (*cb)(struct inode *, struct ceph_cap *,
