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[X,2/4] stop_machine: Disable preemption when waking two stopper threads

Message ID 20190321234412.11113-3-mfo@canonical.com
State New
Headers show
Series LP#1821259 Fix for deadlock in cpu_stopper | expand

Commit Message

Mauricio Faria de Oliveira March 21, 2019, 11:44 p.m. UTC 
From: "Isaac J. Manjarres" <isaacm@codeaurora.org>

BugLink: https://bugs.launchpad.net/bugs/1821259

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

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
CPU.

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
cpu_stop_queue_two_works().

Fixes: 0b26351b910f ("stop_machine, sched: Fix migrate_swap() vs. active_balance() deadlock")
Co-Developed-by: Prasad Sodagudi <psodagud@codeaurora.org>
Signed-off-by: Prasad Sodagudi <psodagud@codeaurora.org>
Co-Developed-by: Pavankumar Kondeti <pkondeti@codeaurora.org>
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: peterz@infradead.org
Cc: matt@codeblueprint.co.uk
Cc: bigeasy@linutronix.de
Cc: gregkh@linuxfoundation.org
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/1530655334-4601-1-git-send-email-isaacm@codeaurora.org
(backported from commit 9fb8d5dc4b649dd190e1af4ead670753e71bf907)
[mfo: backport: refresh context lines]
Signed-off-by: Mauricio Faria de Oliveira <mfo@canonical.com>
---
 kernel/stop_machine.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)
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Patch

diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
index 71435be8bd25..ac4aa744724b 100644
--- a/kernel/stop_machine.c
+++ b/kernel/stop_machine.c
@@ -245,7 +245,11 @@  unlock:
 	spin_unlock_irq(&stopper1->lock);
 	lg_double_unlock(&stop_cpus_lock, cpu1, cpu2);
 
-	wake_up_q(&wakeq);
+	if (!err) {
+		preempt_disable();
+		wake_up_q(&wakeq);
+		preempt_enable();
+	}
 
 	return err;
 }