ceph: create a new session lock to avoid lock inversion

Lockdep was reporting a possible circular lock dependency in
dentry_lease_is_valid().  That function needs to sample the
session's s_cap_gen and and s_cap_ttl fields coherently, but needs
to do so while holding a dentry lock.  The s_cap_lock field was
being used to protect the two fields, but that can't be taken while
holding a lock on a dentry within the session.

In most cases, the s_cap_gen and s_cap_ttl fields only get operated
on separately.  But in three cases they need to be updated together.
Implement a new lock to protect the spots updating both fields
atomically is required.

Signed-off-by: Alex Elder <elder@dreamhost.com>
Reviewed-by: Sage Weil <sage@newdream.net>

1 files changed, 5 insertions, 2 deletions

diff --git a/fs/ceph/mds_client.h b/fs/ceph/mds_client.h
index a50ca0e39475..8c7c04ebb595 100644
--- a/fs/ceph/mds_client.h
+++ b/fs/ceph/mds_client.h
@@ -117,10 +117,13 @@ struct ceph_mds_session {
 	void             *s_authorizer_buf, *s_authorizer_reply_buf;
 	size_t            s_authorizer_buf_len, s_authorizer_reply_buf_len;
 
-	/* protected by s_cap_lock */
-	spinlock_t        s_cap_lock;
+	/* protected by s_gen_ttl_lock */
+	spinlock_t        s_gen_ttl_lock;
 	u32               s_cap_gen;  /* inc each time we get mds stale msg */
 	unsigned long     s_cap_ttl;  /* when session caps expire */
+
+	/* protected by s_cap_lock */
+	spinlock_t        s_cap_lock;
 	struct list_head  s_caps;     /* all caps issued by this session */
 	int               s_nr_caps, s_trim_caps;
 	int               s_num_cap_releases;