linux-kernel - Re: [PATCH RT 3.18] ring-buffer: Mark irq_work as HARD

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Date:	Thu, 16 Apr 2015 16:26:57 +0200
From:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:	Jan Kiszka <jan.kiszka@...mens.com>,
	RT <linux-rt-users@...r.kernel.org>
CC:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH RT 3.18] ring-buffer: Mark irq_work as HARD_IRQ to prevent
 deadlocks

On 04/16/2015 04:06 PM, Jan Kiszka wrote:
> ftrace may trigger rb_wakeups while holding pi_lock which will also be
> requested via trace_...->...->ring_buffer_unlock_commit->...->
> irq_work_queue->raise_softirq->try_to_wake_up. This quickly causes
> deadlocks when trying to use ftrace under -rt.
> 
> Resolve this by marking the ring buffer's irq_work as HARD_IRQ.
> 
> Signed-off-by: Jan Kiszka <jan.kiszka@...mens.com>
> ---
> 
> I'm not yet sure if this doesn't push work into hard-irq context that
> is better not done there on -rt.

everything should be done in the soft-irq.

> 
> I'm also not sure if there aren't more such cases, given that -rt turns
> the default irq_work wakeup policy around. But maybe we are lucky.

The only thing that is getting done in the hardirq is the FULL_NO_HZ
thingy. I would be _very_ glad if we could keep it that way.

>  kernel/trace/ring_buffer.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index f4fbbfc..6a1939e 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -1252,6 +1252,7 @@ rb_allocate_cpu_buffer(struct ring_buffer *buffer, int nr_pages, int cpu)
>  	init_irq_work(&cpu_buffer->irq_work.work, rb_wake_up_waiters);
>  	init_waitqueue_head(&cpu_buffer->irq_work.waiters);
>  	init_waitqueue_head(&cpu_buffer->irq_work.full_waiters);
> +	cpu_buffer->irq_work.work.flags = IRQ_WORK_HARD_IRQ;
>  
>  	bpage = kzalloc_node(ALIGN(sizeof(*bpage), cache_line_size()),
>  			    GFP_KERNEL, cpu_to_node(cpu));
> 

Sebastian
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