linux-kernel - Re: [PATCH 5/7] smp/hotplug: Differentiate the AP completion between up and down

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Date:   Mon, 25 Sep 2017 17:49:35 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...nel.org, tglx@...utronix.de, linux-kernel@...r.kernel.org,
        rostedt@...dmis.org, bigeasy@...utronix.de, efault@....de,
        max.byungchul.park@...il.com, kernel-team@....com
Subject: Re: [PATCH 5/7] smp/hotplug: Differentiate the AP completion between
 up and down

On Wed, Sep 20, 2017 at 07:00:19PM +0200, Peter Zijlstra wrote:
> With lockdep-crossrelease we get deadlock reports that span cpu-up and
> cpu-down chains. Such deadlocks cannot possibly happen because cpu-up
> and cpu-down are globally serialized.
> 
>   takedown_cpu()
>     irq_lock_sparse()
>     wait_for_completion(&st->done)
> 
>                                 cpuhp_thread_fun
>                                   cpuhp_up_callback
>                                     cpuhp_invoke_callback
>                                       irq_affinity_online_cpu
>                                         irq_local_spare()
>                                         irq_unlock_sparse()
>                                   complete(&st->done)
> 
> Now that we have consistent AP state, we can trivially separate the
> AP completion between up and down using st->bringup.

Acked-by: Byungchul Park <byungchul.park@....com>

> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  kernel/cpu.c |   33 ++++++++++++++++++++++++---------
>  1 file changed, 24 insertions(+), 9 deletions(-)
> 
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -46,7 +46,8 @@
>   * @bringup:	Single callback bringup or teardown selector
>   * @cb_state:	The state for a single callback (install/uninstall)
>   * @result:	Result of the operation
> - * @done:	Signal completion to the issuer of the task
> + * @done_up:	Signal completion to the issuer of the task for cpu-up
> + * @done_down:	Signal completion to the issuer of the task for cpu-down
>   */
>  struct cpuhp_cpu_state {
>  	enum cpuhp_state	state;
> @@ -61,7 +62,8 @@ struct cpuhp_cpu_state {
>  	struct hlist_node	*last;
>  	enum cpuhp_state	cb_state;
>  	int			result;
> -	struct completion	done;
> +	struct completion	done_up;
> +	struct completion	done_down;
>  #endif
>  };
>  
> @@ -90,6 +92,18 @@ static void inline cpuhp_lock_release(bo
>  
>  #endif
>  
> +static inline void wait_for_ap_thread(struct cpuhp_cpu_state *st, bool bringup)
> +{
> +	struct completion *done = bringup ? &st->done_up : &st->done_down;
> +	wait_for_completion(done);
> +}
> +
> +static inline void complete_ap_thread(struct cpuhp_cpu_state *st, bool bringup)
> +{
> +	struct completion *done = bringup ? &st->done_up : &st->done_down;
> +	complete(done);
> +}
> +
>  /**
>   * cpuhp_step - Hotplug state machine step
>   * @name:	Name of the step
> @@ -368,7 +382,7 @@ static void __cpuhp_kick_ap(struct cpuhp
>  	smp_mb();
>  	st->should_run = true;
>  	wake_up_process(st->thread);
> -	wait_for_completion(&st->done);
> +	wait_for_ap_thread(st, st->bringup);
>  }
>  
>  static int cpuhp_kick_ap(struct cpuhp_cpu_state *st, enum cpuhp_state target)
> @@ -391,7 +405,7 @@ static int bringup_wait_for_ap(unsigned
>  	struct cpuhp_cpu_state *st = per_cpu_ptr(&cpuhp_state, cpu);
>  
>  	/* Wait for the CPU to reach CPUHP_AP_ONLINE_IDLE */
> -	wait_for_completion(&st->done);
> +	wait_for_ap_thread(st, true);
>  	if (WARN_ON_ONCE((!cpu_online(cpu))))
>  		return -ECANCELED;
>  
> @@ -490,7 +504,8 @@ static void cpuhp_create(unsigned int cp
>  {
>  	struct cpuhp_cpu_state *st = per_cpu_ptr(&cpuhp_state, cpu);
>  
> -	init_completion(&st->done);
> +	init_completion(&st->done_up);
> +	init_completion(&st->done_down);
>  }
>  
>  static int cpuhp_should_run(unsigned int cpu)
> @@ -584,7 +599,7 @@ static void cpuhp_thread_fun(unsigned in
>  	cpuhp_lock_release(bringup);
>  
>  	if (!st->should_run)
> -		complete(&st->done);
> +		complete_ap_thread(st, bringup);
>  }
>  
>  /* Invoke a single callback on a remote cpu */
> @@ -780,7 +795,7 @@ static int takedown_cpu(unsigned int cpu
>  	 *
>  	 * Wait for the stop thread to go away.
>  	 */
> -	wait_for_completion(&st->done);
> +	wait_for_ap_thread(st, false);
>  	BUG_ON(st->state != CPUHP_AP_IDLE_DEAD);
>  
>  	/* Interrupts are moved away from the dying cpu, reenable alloc/free */
> @@ -799,7 +814,7 @@ static void cpuhp_complete_idle_dead(voi
>  {
>  	struct cpuhp_cpu_state *st = arg;
>  
> -	complete(&st->done);
> +	complete_ap_thread(st, false);
>  }
>  
>  void cpuhp_report_idle_dead(void)
> @@ -938,7 +953,7 @@ void cpuhp_online_idle(enum cpuhp_state
>  		return;
>  
>  	st->state = CPUHP_AP_ONLINE_IDLE;
> -	complete(&st->done);
> +	complete_ap_thread(st, true);
>  }
>  
>  /* Requires cpu_add_remove_lock to be held */
>