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From: "tip-bot for Isaac J. Manjarres" <>
Subject: [tip:sched/urgent] stop_machine: Disable preemption when waking two stopper threads
Date: Sun, 15 Jul 2018 03:15:50 -0700	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

Commit-ID:  9fb8d5dc4b649dd190e1af4ead670753e71bf907
Author:     Isaac J. Manjarres <>
AuthorDate: Tue, 3 Jul 2018 15:02:14 -0700
Committer:  Thomas Gleixner <>
CommitDate: Sun, 15 Jul 2018 12:12:45 +0200

stop_machine: Disable preemption when waking two stopper threads

When cpu_stop_queue_two_works() begins to wake the stopper threads, it does
so without preemption disabled, which leads to the following race

The source CPU calls cpu_stop_queue_two_works(), with cpu1 as the source
CPU, and cpu2 as the destination CPU. When adding the stopper threads to
the wake queue used in this function, the source CPU stopper thread is
added first, and the destination CPU stopper thread is added last.

When wake_up_q() is invoked to wake the stopper threads, the threads are
woken up in the order that they are queued in, so the source CPU's stopper
thread is woken up first, and it preempts the thread running on the source

The stopper thread will then execute on the source CPU, disable preemption,
and begin executing multi_cpu_stop(), and wait for an ack from the
destination CPU's stopper thread, with preemption still disabled. Since the
worker thread that woke up the stopper thread on the source CPU is affine
to the source CPU, and preemption is disabled on the source CPU, that
thread will never run to dequeue the destination CPU's stopper thread from
the wake queue, and thus, the destination CPU's stopper thread will never
run, causing the source CPU's stopper thread to wait forever, and stall.

Disable preemption when waking the stopper threads in

Fixes: 0b26351b910f ("stop_machine, sched: Fix migrate_swap() vs. active_balance() deadlock")
Co-Developed-by: Prasad Sodagudi <>
Signed-off-by: Prasad Sodagudi <>
Co-Developed-by: Pavankumar Kondeti <>
Signed-off-by: Pavankumar Kondeti <>
Signed-off-by: Isaac J. Manjarres <>
Signed-off-by: Thomas Gleixner <>

 kernel/stop_machine.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
index f89014a2c238..1ff523dae6e2 100644
--- a/kernel/stop_machine.c
+++ b/kernel/stop_machine.c
@@ -270,7 +270,11 @@ unlock:
 		goto retry;
-	wake_up_q(&wakeq);
+	if (!err) {
+		preempt_disable();
+		wake_up_q(&wakeq);
+		preempt_enable();
+	}
 	return err;

      reply	other threads:[~2018-07-15 10:16 UTC|newest]

Thread overview: 2+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-07-03 22:02 [PATCH v3] " Isaac J. Manjarres
2018-07-15 10:15 ` tip-bot for Isaac J. Manjarres [this message]

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