libceph: avoid KEEPALIVE_PENDING races in ceph_con_keepalive()

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: [e-mail]
Signed-off-by: Ilya Dryomov <[e-mail]>
Tested-by: Myungho Jung <[e-mail]>

net/ceph/messenger.c

···
 	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);