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Date:	Mon, 28 Jul 2014 16:52:01 -0500
From:	Corey Minyard <cminyard@...sta.com>
To:	minyard@....org, rostedt@...dmis.org
CC:	linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org,
	tglx@...utronix.d, C.Emde@...dl.org, bigeasy@...utronix.de,
	jkacur@...hat.com, paul.gortmaker@...driver.com
Subject: Re: [PATCH] tracing: Always run per-cpu ring buffer resize with schedule_work_on()

Ping, I haven't heard anything on this.

-corey

On 07/16/2014 02:07 PM, minyard@....org wrote:
> From: Corey Minyard <cminyard@...sta.com>
>
> The code for resizing the trace ring buffers has to run the per-cpu
> resize on the CPU itself.  The code was using preempt_off() and
> running the code for the current CPU directly, otherwise calling
> schedule_work_on().
>
> At least on RT this could result in the following:
>
> |BUG: sleeping function called from invalid context at kernel/rtmutex.c:673
> |in_atomic(): 1, irqs_disabled(): 0, pid: 607, name: bash
> |3 locks held by bash/607:
> |CPU: 0 PID: 607 Comm: bash Not tainted 3.12.15-rt25+ #124
> |(rt_spin_lock+0x28/0x68)
> |(free_hot_cold_page+0x84/0x3b8)
> |(free_buffer_page+0x14/0x20)
> |(rb_update_pages+0x280/0x338)
> |(ring_buffer_resize+0x32c/0x3dc)
> |(free_snapshot+0x18/0x38)
> |(tracing_set_tracer+0x27c/0x2ac)
>
> probably via
> |cd /sys/kernel/debug/tracing/
> |echo 1 > events/enable ; sleep 2
> |echo 1024 > buffer_size_kb
>
> If we just always use schedule_work_on(), there's no need for the
> preempt_off().  So do that.
>
> Reported-by: Stanislav Meduna <stano@...una.org>
> Signed-off-by: Corey Minyard <cminyard@...sta.com>
> ---
>  kernel/trace/ring_buffer.c | 24 ++++--------------------
>  1 file changed, 4 insertions(+), 20 deletions(-)
>
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 7c56c3d..35825a8 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -1693,22 +1693,14 @@ int ring_buffer_resize(struct ring_buffer *buffer, unsigned long size,
>  			if (!cpu_buffer->nr_pages_to_update)
>  				continue;
>  
> -			/* The update must run on the CPU that is being updated. */
> -			preempt_disable();
> -			if (cpu == smp_processor_id() || !cpu_online(cpu)) {
> +			/* Can't run something on an offline CPU. */
> +			if (!cpu_online(cpu)) {
>  				rb_update_pages(cpu_buffer);
>  				cpu_buffer->nr_pages_to_update = 0;
>  			} else {
> -				/*
> -				 * Can not disable preemption for schedule_work_on()
> -				 * on PREEMPT_RT.
> -				 */
> -				preempt_enable();
>  				schedule_work_on(cpu,
>  						&cpu_buffer->update_pages_work);
> -				preempt_disable();
>  			}
> -			preempt_enable();
>  		}
>  
>  		/* wait for all the updates to complete */
> @@ -1746,22 +1738,14 @@ int ring_buffer_resize(struct ring_buffer *buffer, unsigned long size,
>  
>  		get_online_cpus();
>  
> -		preempt_disable();
> -		/* The update must run on the CPU that is being updated. */
> -		if (cpu_id == smp_processor_id() || !cpu_online(cpu_id))
> +		/* Can't run something on an offline CPU. */
> +		if (!cpu_online(cpu_id))
>  			rb_update_pages(cpu_buffer);
>  		else {
> -			/*
> -			 * Can not disable preemption for schedule_work_on()
> -			 * on PREEMPT_RT.
> -			 */
> -			preempt_enable();
>  			schedule_work_on(cpu_id,
>  					 &cpu_buffer->update_pages_work);
>  			wait_for_completion(&cpu_buffer->update_done);
> -			preempt_disable();
>  		}
> -		preempt_enable();
>  
>  		cpu_buffer->nr_pages_to_update = 0;
>  		put_online_cpus();

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