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[PATCH RT 01/20] sched: reset tasks lockless wake-queues on fork()

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3.18.27-rt27-rc1 stable review patch.
If anyone has any objections, please let me know.

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

From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

In 7675104990ed ("sched: Implement lockless wake-queues") we gained
lockless wake-queues. -RT managed to lockup itself with those. There
could be multiple attempts for task X to enqueue it for a wakeup
_even_ if task X is already running.
The reason is that task X could be runnable but not yet on CPU. The the
task performing the wakeup did not leave the CPU it could performe
multiple wakeups.
With the proper timming task X could be running and enqueued for a
wakeup. If this happens while X is performing a fork() then its its
child will have a !NULL `wake_q` member copied.
This is not a problem as long as the child task does not participate in
lockless wakeups :)

Fixes: 7675104990ed ("sched: Implement lockless wake-queues")
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
---
 kernel/fork.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/fork.c b/kernel/fork.c
index 3a8ecfb38831..1c4edaf4ec36 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -364,6 +364,7 @@ static struct task_struct *dup_task_struct(struct task_struct *orig)
 #endif
 	tsk->splice_pipe = NULL;
 	tsk->task_frag.page = NULL;
+	tsk->wake_q.next = NULL;
 
 	account_kernel_stack(ti, 1);
 
-- 
2.7.0

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[PATCH RT 00/20] Linux 3.18.27-rt27-rc1 Steven Rostedt <rostedt@goodmis.org> - 2016-03-02 16:50 +0100
  [PATCH RT 12/20] tick/broadcast: Make broadcast hrtimer irqsafe Steven Rostedt <rostedt@goodmis.org> - 2016-03-02 16:50 +0100
  [PATCH RT 01/20] sched: reset tasks lockless wake-queues on fork() Steven Rostedt <rostedt@goodmis.org> - 2016-03-02 16:50 +0100
  [PATCH RT 04/20] preempt-lazy: Add the lazy-preemption check to preempt_schedule() Steven Rostedt <rostedt@goodmis.org> - 2016-03-02 16:50 +0100
  [PATCH RT 09/20] kernel: migrate_disable() do fastpath in atomic & irqs-off Steven Rostedt <rostedt@goodmis.org> - 2016-03-02 16:50 +0100

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