[PATCH 4.9 21/24] libceph: avoid KEEPALIVE_PENDING races in ceph_con_keepalive()

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Ilya Dryomov <idryomov@gmail.com>

commit 4aac9228d16458cedcfd90c7fb37211cf3653ac3 upstream.

con_fault() can transition the connection into STANDBY right after
ceph_con_keepalive() clears STANDBY in clear_standby():

    libceph user thread               ceph-msgr worker

ceph_con_keepalive()
  mutex_lock(&con->mutex)
  clear_standby(con)
  mutex_unlock(&con->mutex)
                                mutex_lock(&con->mutex)
                                con_fault()
                                  ...
                                  if KEEPALIVE_PENDING isn't set
                                    set state to STANDBY
                                  ...
                                mutex_unlock(&con->mutex)
  set KEEPALIVE_PENDING
  set WRITE_PENDING

This triggers warnings in clear_standby() when either ceph_con_send()
or ceph_con_keepalive() get to clearing STANDBY next time.

I don't see a reason to condition queue_con() call on the previous
value of KEEPALIVE_PENDING, so move the setting of KEEPALIVE_PENDING
into the critical section -- unlike WRITE_PENDING, KEEPALIVE_PENDING
could have been a non-atomic flag.

Reported-by: syzbot+acdeb633f6211ccdf886@syzkaller.appspotmail.com
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Tested-by: Myungho Jung <mhjungk@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 net/ceph/messenger.c |    5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

--- a/net/ceph/messenger.c
+++ b/net/ceph/messenger.c
@@ -3186,9 +3186,10 @@ void ceph_con_keepalive(struct ceph_conn
 	dout("con_keepalive %p\n", con);
 	mutex_lock(&con->mutex);
 	clear_standby(con);
+	con_flag_set(con, CON_FLAG_KEEPALIVE_PENDING);
 	mutex_unlock(&con->mutex);
-	if (con_flag_test_and_set(con, CON_FLAG_KEEPALIVE_PENDING) == 0 &&
-	    con_flag_test_and_set(con, CON_FLAG_WRITE_PENDING) == 0)
+
+	if (con_flag_test_and_set(con, CON_FLAG_WRITE_PENDING) == 0)
 		queue_con(con);
 }
 EXPORT_SYMBOL(ceph_con_keepalive);