Date: Thu, 16 Apr 2015 10:12:00 -0400
From: Steven Rostedt <rostedt@goodmis.org>
To: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        RT <linux-rt-users@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH RT 3.18] ring-buffer: Mark irq_work as HARD_IRQ to
 prevent deadlocks
Message-ID: <20150416101200.0633e93d@gandalf.local.home>
In-Reply-To: <552FC1FE.4020406@siemens.com>
References: <552FC1FE.4020406@siemens.com>
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On Thu, 16 Apr 2015 16:06:54 +0200
Jan Kiszka <jan.kiszka@siemens.com> 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.

Hmm,  I could have sworn I wrote a patch like this not too long ago.

I'll have to check that out.

Thanks,

-- Steve

> 
> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
> ---
> 
> I'm not yet sure if this doesn't push work into hard-irq context that
> is better not done there on -rt.
> 
> 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.
> 
>  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));

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