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From: Sebastian Andrzej Siewior <>
To: Peter Zijlstra <>
Cc: Davidlohr Bueso <>,
	Thomas Gleixner <>,
	Steven Rostedt <>,, Ingo Molnar <>
Subject: [PATCH] sched: reset task's lockless wake-queues on fork()
Date: Mon, 21 Dec 2015 18:17:10 +0100
Message-ID: <> (raw)

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 <>
 kernel/fork.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/fork.c b/kernel/fork.c
index b508f757c60b..46c1e8342ad8 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -393,6 +393,7 @@ static struct task_struct *dup_task_struct(struct task_struct *orig)
 	tsk->splice_pipe = NULL;
 	tsk-> = NULL;
+	tsk-> = NULL;
 	account_kernel_stack(ti, 1);

             reply index

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-12-21 17:17 Sebastian Andrzej Siewior [this message]
2016-01-06 18:48 ` [tip:sched/core] sched/core: Reset task' s " tip-bot for Sebastian Andrzej Siewior
2016-01-08  9:41   ` Thomas Gleixner
2016-01-09 19:25     ` Davidlohr Bueso

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