Re: [PATCH RT 3.18] ring-buffer: Mark irq_work as HARD_IRQ to prevent deadlocks

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 04/16/2015 04:28 PM, Jan Kiszka wrote:
> On 2015-04-16 16:26, Sebastian Andrzej Siewior wrote:
>> 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@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.
>>
>> 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.
> 
> Then - to my current understanding - we need an NMI-safe trigger for
> soft-irq work. Is there anything like this existing already? Or can we
> still use the IPI-based kick without actually doing the work in hard-irq
> context?

But if you trigger it via IPI it will still run in hardirq context,
right? Can you describe how run into this and try to think about it in
a quiet moment. It it just enabling the function tracer and running it?

> Jan

Sebastian