Hello Sebastian,
On Tue, Mar 23, 2021 at 10:04:13AM +0100, Sebastian Andrzej Siewior wrote:
> On 2021-03-23 08:34:47 [+0100], Uwe Kleine-König wrote:
> > On Mon, Mar 22, 2021 at 09:48:36PM +0100, Sebastian Andrzej Siewior wrote:
> > > On 2021-03-22 14:40:32 [+0100], Uwe Kleine-König wrote:
> > > > 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.
> > >
> > > Uwe, the patch reverts a change which was needed for !RT + threadirqs.
> >
> > This would be a useful information for the commit log.
> >
> > > The commit message claims that since the referenced commit "… interrupt
> > > handlers always run with interrupts disabled on non-RT… ". This has
> > > nothing to do with _this_ change. It argues why the workaround is not
> > > needed.
> >
> > It argues why the work around is not needed on non-RT. It might be
> > obvious for someone who is firm in the RT concepts, but IMHO commit logs
> > should be understandable by and make sense for a wider audience than the
> > deep experts. From what I know about RT "Force-threaded interrupt
> > handlers used to run with interrupts enabled" still applies there.
>
> Yes. The commit Johan referenced explains it in more detail.
In my book the commit log should be understandable without reading the
referenced commits.
> > > If the referenced commit breaks RT then this is another story.
> >
> > I'm surprised to hear that from you. With the goal to get RT into
> > mainline I would expect you to be happy if people consider the effects
> > on RT in their reviews.
>
> Correct, I do and I am glad if people consider other aspects of the
> kernel in their review including RT.
>
> > > > 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.
> > >
> > > There was never reason (or a lockdep splat) for it on RT. If so you
> > > should have seen it, right?
> >
> > No, I don't consider myself to be an RT expert who is aware of all the
> > problems. So I admit that for me the effect on RT of the patch under
> > discussion isn't obvious. I just wonder that the change is justified
> > with being OK on non-RT. So it's either bad that it breaks RT *or*
> > improving the commit log would be great.
> >
> > And even if I had reason to believe that there is no problem with the
> > commit on RT, I'd still wish that the commit log wouldn't suggest to the
> > casual reader that there might be a problem.
>
> Okay. I added a sentence. What about this rewording:
>
> 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.
> RT is not affected by the referenced commit and the workaround, that is
> reverted, was not required because spinlock_t must not be acquired on
> RT in hardirq context.
>
> Drop the now obsolete workaround added by commit 33f16855dcb9 ("tty:
> serial: imx: fix potential deadlock").
This resolves my concerns. Thanks
Uwe
--
Pengutronix e.K. | Uwe Kleine-König |
Industrial Linux Solutions | https://www.pengutronix.de/ |
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