linux-kernel - Re: [PATCH v4 08/24] seqlock: lockdep assert non-preemptibility on seqcount

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Date:   Sun, 9 Aug 2020 20:42:51 +0200
From:   "Ahmed S. Darwish" <a.darwish@...utronix.de>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        "Sebastian A. Siewior" <bigeasy@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 08/24] seqlock: lockdep assert non-preemptibility on
 seqcount_t write

On Sat, Aug 08, 2020 at 04:21:22PM -0700, Guenter Roeck wrote:
> On Mon, Jul 20, 2020 at 05:55:14PM +0200, Ahmed S. Darwish wrote:
> > Preemption must be disabled before entering a sequence count write side
> > critical section.  Failing to do so, the seqcount read side can preempt
> > the write side section and spin for the entire scheduler tick.  If that
> > reader belongs to a real-time scheduling class, it can spin forever and
> > the kernel will livelock.
> >
> > Assert through lockdep that preemption is disabled for seqcount writers.
> >
>
> This patch is causing compile failures for various images (eg arm:allmodconfig,
> arm:imx_v6_v7_defconfig, mips:allmodconfig).
>
> In file included from arch/arm/include/asm/bug.h:60,
>                  from include/linux/bug.h:5,
>                  from include/linux/thread_info.h:12,
>                  from include/asm-generic/current.h:5,
>                  from ./arch/arm/include/generated/asm/current.h:1,
>                  from include/linux/sched.h:12,
>                  from arch/arm/kernel/asm-offsets.c:11:
> include/linux/seqlock.h: In function 'write_seqcount_begin_nested':
> include/asm-generic/percpu.h:31:40: error: implicit declaration of function 'raw_smp_processor_id'
>
> Reverting it fixes the problem. Is this being addressed ?
>

@Peter, I think let's revert this one for now?

Shall I also send you a version of:

    [PATCH v4 09/24] seqlock: Extend seqcount API with associated locks
    https://lkml.kernel.org/r/20200720155530.1173732-10-a.darwish@linutronix.de/

with the problematic "lockdep_assert_preemption_disabled()" removed?

> Guenter
>

Thanks,

--
Ahmed S. Darwish
Linutronix GmbH