linux-kernel - Re: [PATCH] serial: imx: drop workaround for forced irq threading

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Message-ID: <20210324112626.iqteqgfbkci3xmvh@pengutronix.de>
Date:   Wed, 24 Mar 2021 12:26:26 +0100
From:   Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To:     Johan Hovold <johan@...nel.org>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
        Sam Nobs <samuel.nobs@...tradio.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] serial: imx: drop workaround for forced irq threading

Hello Johan,

On Tue, Mar 23, 2021 at 03:37:29PM +0100, Johan Hovold wrote:
> On Mon, Mar 22, 2021 at 02:40:32PM +0100, Uwe Kleine-König wrote:
> > On Mon, Mar 22, 2021 at 02:20:57PM +0100, Johan Hovold wrote:
> > > On Mon, Mar 22, 2021 at 12:55:36PM +0100, Uwe Kleine-König wrote:
> > > > On Mon, Mar 22, 2021 at 12:39:18PM +0100, Sebastian Andrzej Siewior wrote:
> > > > > On 2021-03-22 12:34:02 [+0100], Uwe Kleine-König wrote:
> > > > > > On Mon, Mar 22, 2021 at 12:10:36PM +0100, Johan Hovold wrote:
> > > > > > > Force-threaded interrupt handlers used to run with interrupts enabled,
> > > > > > > something which could lead to deadlocks in case a threaded handler
> > > > > > > shared a lock with code running in hard interrupt context (e.g. timer
> > > > > > > callbacks) and did not explicitly disable interrupts.
> > > > > > > 
> > > > > > > This was specifically the case for serial drivers that take the port
> > > > > > > lock in their console write path as printk can be called from hard
> > > > > > > interrupt context also with forced threading ("threadirqs").
> > > > > > > 
> > > > > > > Since commit 81e2073c175b ("genirq: Disable interrupts for force
> > > > > > > threaded handlers") interrupt handlers always run with interrupts
> > > > > > > disabled on non-RT so that drivers no longer need to do handle this.
> > > > > > 
> > > > > > So we're breaking RT knowingly here? If this is the case I'm not happy
> > > > > > with your change. (And if RT is not affected a different wording would
> > > > > > be good.)
> > > > > 
> > > > > Which wording, could you be more specific? It looks good from here and
> > > > > no, RT is not affected.
> > > > 
> > > > The commit log says essentially: "The change is fine on non-RT" which
> > > > suggests there is a problem on RT.
> > > 
> > > I don't think you can read that into the commit message.
> > 
> > From a strictly logically point of view you indeed cannot. But if you go
> > to the street and say to people there that they can park their car in
> > this street free of charge between Monday and Friday, I expect that most
> > of them will assume that they have to pay for parking on weekends.
> 
> That analogy would almost seem to suggest bad intent on my side.

That analogy's purpose was to put over my point that writing
(paraphrased) "Since non-RT changed, this workaround isn't necessary any
more" suggests to me that the change might be bad for RT. So again,
there was no harm intended, this is just a call for clearing up either
the commit log to make it obvious the change is right or to fix the
problem on RT if there is any.

> To say that this workaround is no longer needed on !RT does not imply
> that it is needed on RT. If anything it suggests I have considered RT,
> I'd say.

The code in question was used for both RT and non-RT. You drop it for
both cases and only justify one of them. OK, fine, you considered both
cases. Just from reading the commit log I considered you didn't. It's
completely ok for me to be wrong here, but I still think the chosen
words are not optimal and stumbling as I did is easy. So I still see a
potential to improve the wording.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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