Re: [PATCH 1/2] sched/idle: Fix arch_cpu_idle() vs tracing

Will Deacon
2020-12-01T13:39:34Z

On Tue, Dec 01, 2020 at 01:52:46PM +0100, Peter Zijlstra wrote:
> On Tue, Dec 01, 2020 at 12:56:27PM +0100, Sven Schnelle wrote:
> > Peter Zijlstra <peterz@infradead.org> writes:
> > > On Mon, Nov 30, 2020 at 01:00:03PM -0800, Guenter Roeck wrote:
> > >> On Fri, Nov 20, 2020 at 12:41:46PM +0100, Peter Zijlstra wrote:
> > >> > We call arch_cpu_idle() with RCU disabled, but then use
> > >> > local_irq_{en,dis}able(), which invokes tracing, which relies on RCU.
> > >> >
> > >> > Switch all arch_cpu_idle() implementations to use
> > >> > raw_local_irq_{en,dis}able() and carefully manage the
> > >> > lockdep,rcu,tracing state like we do in entry.
> > >> >
> > >> > (XXX: we really should change arch_cpu_idle() to not return with
> > >> > interrupts enabled)
> > >> >
> > >>
> > >> Has this patch been tested on s390 ? Reason for asking is that it causes
> > >> all my s390 emulations to crash. Reverting it fixes the problem.
> > >
> > > My understanding is that it changes the error on s390. Previously it
> > > would complain about the local_irq_enable() in arch_cpu_idle(), now it
> > > complains when taking an interrupt during idle.
> >
> > I looked into adding the required functionality for s390, but the code
> > we would need to add to entry.S is rather large - as you noted we would
> > have to duplicate large portions of irqentry_enter() into our code.
> > Given that s390 was fine before that patch, can you revert it and submit
> > it again during the next merge window?
>
> I'm not sure I understand how s390 was fine without it, let me consdier.
> Also, what's the status of ARM64, they do need this too.

We've got the batch of fixes from Mark queued for -rc7:

https://fixes.arm64.dev/

which rely on Peter's patch:

https://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git/commit/?h=for-next/fixes&id=114e0a684753516ef4b71ccb55a8ebcfa8735edb

There's room for consolidation and cleanup in future, but right now we've
focussed purely on fixing things.

Will