linux-kernel - Re: [PATCH] s390/idle: Fix suspicious RCU usage

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Message-ID: <20201007100551.GC2628@hirez.programming.kicks-ass.net>
Date:   Wed, 7 Oct 2020 12:05:51 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sven Schnelle <svens@...ux.ibm.com>
Cc:     hca@...ux.ibm.com, linux-kernel@...r.kernel.org,
        linux-s390@...r.kernel.org, rafael.j.wysocki@...el.com
Subject: Re: [PATCH] s390/idle: Fix suspicious RCU usage

On Wed, Oct 07, 2020 at 09:53:25AM +0200, Sven Schnelle wrote:
> Hi Peter,
> 
> peterz@...radead.org writes:
> 
> > After commit eb1f00237aca ("lockdep,trace: Expose tracepoints") the
> > lock tracepoints are visible to lockdep and RCU-lockdep is finding a
> > bunch more RCU violations that were previously hidden.
> >
> > Switch the idle->seqcount over to using raw_write_*() to avoid the
> > lockdep annotation and thus the lock tracepoints.
> >
> > Reported-by: Guenter Roeck <linux@...ck-us.net>
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > [..]
> 
> I'm still seeing the splat below on s390 when irq tracing is enabled:

Damn... :/

This one is tricky, trouble seems to be that arch_cpu_idle() is defined
to enable interrupts (no doubt because ot x86 :/), but we call it before
rcu_exit_idle().

What a mess... let me rummage around the various archs to see what makes
most sense here.