linux-kernel - Question pertaining to request_threaded

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Message-ID: <CAEtk5gfd1CpPdaF4ufjzZzR2pZYKb9cvKUwsH15XUGcNCP+WFA@mail.gmail.com>
Date:	Wed, 20 Mar 2013 07:53:52 -0700
From:	Vijay Dixit <thelonejoker@...il.com>
To:	linux-kernel@...r.kernel.org
Subject: Question pertaining to request_threaded_irq

 Hello,

I am new to the Kernel-Mailing list. I am not subscribed at the moment
and would really appreciate it, if I can be CC'd for all the
reply/responses for my question.

I have searched all over the web but haven't found a convincing answer
to a couple of related questions I have, with regard to the
"request_threaded_irq" feature.

QUESTION-1
****************
Firstly, I was reading this article, regarding threaded IRQ's:

http://lwn.net/Articles/302043/

and there is this one line that isn't clear to me:

"Converting an interrupt to threaded makes only sense when the handler
code takes advantage of it by integrating tasklet/softirq
functionality and simplifying the locking."

I understand had we gone ahead with a "traditional", top half/bottom
half approach, we would have needed either spin-locks or disable local
IRQ to meddle with shared data. But, what I don't understand is, how
would threaded interrupts simplify the need for locking by integrating
tasklet/softirq functionality.

QUESTION-2
****************
Secondly, what advantage (if any), does a request_threaded_handler
approach have over a work_queue based bottom half approach ? In both
cases it seems, as though the "work" is deferred to a dedicated
thread. So, what is the difference ?

QUESTION-3
****************
Lastly, in the following prototype:

int request_threaded_irq(unsigned int irq, irq_handler_t handler,
irq_handler_t thread_fn, unsigned long irqflags, const char *devname,
void *dev_id)

Is it possible that the "handler" part of the IRQ is continuously
triggered by the relevant IRQ (say a UART receving characters at a
high rate), even while the "thread_fn"(writing rx'd bytes to a
circular buffer) part of the interrupt handler is busy processing
IRQ's from previous wakeups ? So, wouldn't the handler be trying to
"wakeup" an already running "thread_fn" ? How would the running irq
"thread_fn" behave in that case ?

I would really appreciate if someone can help me understand this.

Thanks,
vj
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