Fix the deadlock of rl_lock.

Fix the deadlock of rl_lock.

Hi,

Changelog of v2:
  - Add the description of the problematic call sequence.
  - There is not any change in the code.

[PATCH-v2] Fix the deadlock of rl_lock.

For avoiding the deadlock, protect cdgroup_change_cgroup_uid_gid_flags()
by blocking SIGUSR2 signal.

The problematic call sequence is the following:
----------------------------------------------------------------------
* CGRULESENGD DAEMON *
  << cgre_flash_rules() is the signal handler for SIGUSR2 signal >>
  cgre_create_netlink_socket_process_msg()
    << Receive a UID/GID event packet >>
    cgre_handle_msg()
      cgre_process_event()
        cgroup_change_cgroup_uid_gid_flags()
          cgroup_find_matching_rule_uid_gid()
            pthread_rwlock_wrlock(&rl_lock);  << Get the lock of rl_lock >>
            << Receive a SIGUSR2 signal, and switch to cgre_flash_rules() >>
  cgre_flash_rules()
    cgroup_reload_cached_rules()
      cgroup_parse_rules()
        pthread_rwlock_wrlock(&rl_lock);  << deadlock ! >>
----------------------------------------------------------------------

A cgrulesengd daemon needs a lock of rl_lock for referring configuration
buffer. On the other way, the daemon reloads configuration file when
receiving SIGUSR2 signal, and it needs the same lock in cgroup_parse_rules().
So cgroup_change_cgroup_uid_gid_flags() should be protected from SIGUSR2
signal for avoiding the deadlock.

Thanks
Ken'ichi Ohmichi

Signed-off-by: Ken'ichi Ohmichi <oomichi@mxs.nes.nec.co.jp>
Signed-off-by: Dhaval Giani <dhaval@linux.vnet.ibm.com>