Re: [PATCH v4 08/24] seqlock: lockdep assert non-preemptibility on seqcount_t write

Ahmed S. Darwish
2020-08-09T18:42:53Z

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