linux-kernel - Re: [PATCH] genirq: Keep handle_nested_irq() from touching desc->threads

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Date: Tue, 11 Jun 2024 21:19:57 +0800
From: Peng Liu <iwtbavbm@...il.com>
To: Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
	stable@...r.kernel.org
Cc: gregkh@...uxfoundation.org, maz@...nel.org, iwtbavbm@...il.com
Subject: Re: [PATCH] genirq: Keep handle_nested_irq() from touching desc->threads_active

On Mon, Jun 10, 2024 at 08:23:09PM +0200, Thomas Gleixner wrote:
> On Mon, Jun 10 2024 at 02:30, Peng Liu wrote:
> > handle_nested_irq() is supposed to be running inside the parent thread
> > handler context. It per se has no dedicated kernel thread, thus shouldn't
> > touch desc->threads_active. The parent kernel thread has already taken
> > care of this.
> 
> No it has not. The parent thread has marked itself in the parent threads
> interrupt descriptor.
> 
> How does that help synchronizing the nested interrupt, which has a
> separate interrupt descriptor?

Right, I never thought there would be more than one interrupt
descriptors involved which is quite common.

> 
> > Fixes: e2c12739ccf7 ("genirq: Prevent nested thread vs synchronize_hardirq() deadlock")
> > Cc: stable@...r.kernel.org
> 
> There is nothing to fix.
> 
> > Signed-off-by: Peng Liu <iwtbavbm@...il.com>
> > ---
> >
> > Despite of its correctness, I'm afraid the testing on my only PC can't
> > cover the affected code path. So the patch may be totally -UNTESTED-.
> 
> Which correctness?
> 
> The change log of the commit you want to "fix" says:
> 
>     Remove the incorrect usage in the nested threaded interrupt case and
>     instead re-use the threads_active / wait_for_threads mechanism to
>     wait for nested threaded interrupts to complete.
> 
> It's very clearly spelled out, no?

Indeed, due to my ignorance, I never thought there might be more
descriptors involved. Now think about it, I never really understood
the meaning of the above change log.

Thanks for your time and concise explanation.

Peng

>
> Thanks,
> 
>         tglx