linux-kernel - Re: [PATCH v3 10/14] locking/percpu-rwsem: Add percpu_is_write_locked() and percpu_is_read

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Message-ID: <CANpmjNPSOnMN3Fc4yxoArTytQcW4n6sPCN-LnisTn58xLU=4HA@mail.gmail.com>
Date:   Mon, 29 Aug 2022 08:00:00 +0200
From:   Marco Elver <elver@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Frederic Weisbecker <frederic@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        linuxppc-dev@...ts.ozlabs.org, linux-perf-users@...r.kernel.org,
        x86@...nel.org, linux-sh@...r.kernel.org,
        kasan-dev@...glegroups.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 10/14] locking/percpu-rwsem: Add percpu_is_write_locked()
 and percpu_is_read_locked()

On Wed, 17 Aug 2022 at 14:48, Peter Zijlstra <peterz@...radead.org> wrote:
> On Mon, Jul 04, 2022 at 05:05:10PM +0200, Marco Elver wrote:
> > +bool percpu_is_read_locked(struct percpu_rw_semaphore *sem)
> > +{
> > +     return per_cpu_sum(*sem->read_count) != 0;
> > +}
> > +EXPORT_SYMBOL_GPL(percpu_is_read_locked);
>
> I don't think this is correct; read_count can have spurious increments.
>
> If we look at __percpu_down_read_trylock(), it does roughly something
> like this:
>
>         this_cpu_inc(*sem->read_count);
>         smp_mb();
>         if (!sem->block)
>                 return true;
>         this_cpu_dec(*sem->read_count);
>         return false;
>
> So percpu_is_read_locked() needs to ensure the read_count is non-zero
> *and* that block is not set.

I shall go and fix. v4 incoming (if more comments before that, please shout).

> That said; I really dislike the whole _is_locked family with a passion.
> Let me try and figure out what you need this for.

As in the other email, it's for the dbg_*() functions for kgdb's
benefit (avoiding deadlock if kgdb wants a breakpoint, while we're in
the process of handing out a breakpoint elsewhere and have the locks
taken).

Thanks,
-- Marco