linux-kernel - Re: cpu stopper threads and setaffinity leads to deadlock

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Message-ID: <20180802094908.GK2494@hirez.programming.kicks-ass.net>
Date:   Thu, 2 Aug 2018 11:49:08 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sodagudi Prasad <psodagud@...eaurora.org>
Cc:     mingo@...nel.org, gregkh@...uxfoundation.org,
        bigeasy@...utronix.de, tglx@...utronix.de, isaacm@...eaurora.org,
        linux-kernel@...r.kernel.org
Subject: Re: cpu stopper threads and setaffinity leads to deadlock

On Wed, Aug 01, 2018 at 06:34:40PM -0700, Sodagudi Prasad wrote:
> Due to cross migration of tasks between cpu7 and cpu3, migration/7 has
> started executing and waits for the migration/3 task, so that they can
> proceed within the multi cpu stop state machine together.
> Unfortunately stress-ng-affin is affine to cpu7, and since migration 7 has
> started running, and has monopolized cpu7’s execution, stress-ng will never
> run on cpu7, and cpu3’s migration task is never woken up.

> diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
> index e190d1e..f932e1e 100644
> --- a/kernel/stop_machine.c
> +++ b/kernel/stop_machine.c
> @@ -87,9 +87,9 @@ static bool cpu_stop_queue_work(unsigned int cpu, struct
> cpu_stop_work *work)
>                 __cpu_stop_queue_work(stopper, work, &wakeq);
>         else if (work->done)
>                 cpu_stop_signal_done(work->done);
> -       raw_spin_unlock_irqrestore(&stopper->lock, flags);
> 
>         wake_up_q(&wakeq);
> +       raw_spin_unlock_irqrestore(&stopper->lock, flags);
> 

So why didn't you do the 'obvious' parallel to what you did for
cpu_stop_queue_two_works(), namely:

--- a/kernel/stop_machine.c
+++ b/kernel/stop_machine.c
@@ -81,6 +81,7 @@ static bool cpu_stop_queue_work(unsigned
 	unsigned long flags;
 	bool enabled;
 
+	preempt_disable();
 	raw_spin_lock_irqsave(&stopper->lock, flags);
 	enabled = stopper->enabled;
 	if (enabled)
@@ -90,6 +91,7 @@ static bool cpu_stop_queue_work(unsigned
 	raw_spin_unlock_irqrestore(&stopper->lock, flags);
 
 	wake_up_q(&wakeq);
+	preempt_enable();
 
 	return enabled;
 }