Since the tasklist is protected by rcu list operations, it is safe
to convert the read_lock()s to rcu_read_lock().
Suggested-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: NMandeep Singh Baines <msb@google.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Impact: extend the scope of hung-task checks

Changed the default value of hung_task_check_count to PID_MAX_LIMIT.
hung_task_batch_count added to put an upper bound on the critical
section. Every hung_task_batch_count checks, the rcu lock is never
held for a too long time.

Keeping the critical section small minimizes time preemption is disabled
and keeps rcu grace periods small.

To prevent following a stale pointer, get_task_struct is called on g and t.
To verify that g and t have not been unhashed while outside the critical
section, the task states are checked.

The design was proposed by Frédéric Weisbecker.
Signed-off-by: NMandeep Singh Baines <msb@google.com>
Suggested-by: NFrédéric Weisbecker <fweisbec@gmail.com>
Acked-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Impact: fix potential false panic

A potential race exists if sysctl_hung_task_timeout_secs is reset to 0
while inside check_hung_uniterruptible_tasks(). If check_task() is
entered, a comparison with 0 will result in a false hung_task being
detected.

If sysctl_hung_task_panic is set, the system will panic.
Signed-off-by: NMandeep Singh Baines <msb@google.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Decoupling allows:

* hung tasks check to happen at very low priority

* hung tasks check and softlockup to be enabled/disabled independently
  at compile and/or run-time

* individual panic settings to be enabled disabled independently
  at compile and/or run-time

* softlockup threshold to be reduced without increasing hung tasks
  poll frequency (hung task check is expensive relative to softlock watchdog)

* hung task check to be zero over-head when disabled at run-time
Signed-off-by: NMandeep Singh Baines <msb@google.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>