linux-kernel - [PATCH] sched: reset task's lockless wake-queues on fork()

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Message-ID: <20151221171710.GA5499@linutronix.de>
Date:	Mon, 21 Dec 2015 18:17:10 +0100
From:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Davidlohr Bueso <dbueso@...e.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>
Subject: [PATCH] sched: reset task's lockless wake-queues on fork()

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@...utronix.de>
---
 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)
 #endif
 	tsk->splice_pipe = NULL;
 	tsk->task_frag.page = NULL;
+	tsk->wake_q.next = NULL;

 	account_kernel_stack(ti, 1);

-- 
2.6.4

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