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

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Message-ID: <20210322134032.kmirudtnkd4akkgu@pengutronix.de>
Date:   Mon, 22 Mar 2021 14:40:32 +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 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.

So when you said that on on-RT the reason why it used to need a
workaround is gone made me wonder what that implies for RT.

> Why would I knowingly break RT?

No offence intended. Priorities are different for different maintainers
and I wouldn't bet that all maintainers care for RT.

Best regards
Uwe

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

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