linux-kernel - Re: [RFC] genirq: Flush the irq thread on synchronization

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Date:	Sun, 4 Dec 2011 21:09:32 +0200
From:	Ido Yariv <ido@...ery.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [RFC] genirq: Flush the irq thread on synchronization

Hi Thomas,

On Sat, Dec 03, 2011 at 12:21:46AM +0100, Thomas Gleixner wrote:
> I can see your problem, but this might lead to threads_active leaks
> under certain conditions. desc->threads_active was only meant to deal
> with shared interrupts.
> 
> We explicitely allow a design where the primary handler can leave the
> device interrupt enabled and allow further interrupts to occur while
> the handler is running. We only have a single bit to note that the
> thread should run, but your wakeup would up the threads_active count
> in that scenario several times w/o a counterpart which decrements it.
> 
> The solution for this is to keep the current threads_active semantics
> and make the wait function different. Instead of waiting for
> threads_active to become 0 it should wait for threads_active == 0 and
> the IRQTF_RUNTHREAD for all actions to be cleared. To avoid looping
> over the actions, we can take a similar approach as we take with the
> desc->threads_oneshot bitfield.

Thanks for reviewing this.

I might be missing something, but I don't see any potential
threads_active leaks in this approach. We wont increase threads_active
if IRQTF_RUNTHREAD was already set beforehand (as test_and_set_bit()
will return 1).

If irq_wake_thread is called multiple times before irq_thread has had a
chance to run, threads_active will only be increased once and decreased
back when IRQTF_RUNTHREAD is cleared.

Am I missing something? If not, do you see any other issues with this
implementation?

Thanks,
Ido.
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