ceph: ceph_kick_flushing_caps needs the s_mutex

The mdsc->cap_dirty_lock is not held while walking the list in
ceph_kick_flushing_caps, which is not safe.

ceph_early_kick_flushing_caps does something similar, but the
s_mutex is held while it's called and I think that guards against
changes to the list.

Ensure we hold the s_mutex when calling ceph_kick_flushing_caps,
and add some clarifying comments.
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

fs/ceph/caps.c

@@ -2518,6 +2518,8 @@ void ceph_kick_flushing_caps(struct ceph_mds_client *mdsc,
 	struct ceph_cap *cap;
 	u64 oldest_flush_tid;
 
+	lockdep_assert_held(&session->s_mutex);
+
 	dout("kick_flushing_caps mds%d\n", session->s_mds);
 
 	spin_lock(&mdsc->cap_dirty_lock);

fs/ceph/mds_client.c

@@ -4024,7 +4024,9 @@ static void check_new_map(struct ceph_mds_client *mdsc,
 			    oldstate != CEPH_MDS_STATE_STARTING)
 				pr_info("mds%d recovery completed\n", s->s_mds);
 			kick_requests(mdsc, i);
+			mutex_lock(&s->s_mutex);
 			ceph_kick_flushing_caps(mdsc, s);
+			mutex_unlock(&s->s_mutex);
 			wake_up_session_caps(s, RECONNECT);
 		}
 	}

fs/ceph/mds_client.h

@@ -199,8 +199,10 @@ struct ceph_mds_session {
 	struct list_head  s_cap_releases; /* waiting cap_release messages */
 	struct work_struct s_cap_release_work;
 
-	/* both protected by s_mdsc->cap_dirty_lock */
+	/* See ceph_inode_info->i_dirty_item. */
 	struct list_head  s_cap_dirty;	      /* inodes w/ dirty caps */
+
+	/* See ceph_inode_info->i_flushing_item. */
 	struct list_head  s_cap_flushing;     /* inodes w/ flushing caps */
 
 	unsigned long     s_renew_requested; /* last time we sent a renew req */

fs/ceph/super.h

@@ -362,8 +362,11 @@ struct ceph_inode_info {
 	struct list_head i_dirty_item;
 
 	/*
-	 * Link to session's s_cap_flushing list. Protected by
-	 * mdsc->cap_dirty_lock.
+	 * Link to session's s_cap_flushing list. Protected in a similar
+	 * fashion to i_dirty_item, but also by the s_mutex for changes. The
+	 * s_cap_flushing list can be walked while holding either the s_mutex
+	 * or msdc->cap_dirty_lock. List presence can also be checked while
+	 * holding the i_ceph_lock for this inode.
 	 */
 	struct list_head i_flushing_item;