[PATCH 4.2.y-ckt 304/305] sched/core: Reset task's lockless wake-queues on fork()

[Kamal Mostafa]

4.2.8-ckt2 -stable review patch.  If anyone has any objections, please let me know.

---8<------------------------------------------------------------

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

commit 093e5840ae76f1082633503964d035f40ed0216d upstream.

In the following commit:

  7675104990ed ("sched: Implement lockless wake-queues")

we gained lockless wake-queues.

The -RT kernel 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 :)

Signed-off-by: Sebastian Andrzej Siewior <bigeasy at linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz at infradead.org>
Cc: Davidlohr Bueso <dbueso at suse.de>
Cc: Linus Torvalds <torvalds at linux-foundation.org>
Cc: Peter Zijlstra <peterz at infradead.org>
Cc: Steven Rostedt <rostedt at goodmis.org>
Cc: Thomas Gleixner <tglx at linutronix.de>
Fixes: 7675104990ed ("sched: Implement lockless wake-queues")
Link: http://lkml.kernel.org/r/20151221171710.GA5499@linutronix.de
Signed-off-by: Ingo Molnar <mingo at kernel.org>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 kernel/fork.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/fork.c b/kernel/fork.c
index e769c8c..a0fb0ce 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -379,6 +379,7 @@ 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);
 
-- 
1.9.1