linux-kernel - Re: [PATCH] notifier: Make atomic_notifiers use raw

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Message-ID: <20210806182049.GC2184@worktop.programming.kicks-ass.net>
Date:   Fri, 6 Aug 2021 20:20:49 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Ingo Molnar <mingo@...nel.org>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
Subject: Re: [PATCH] notifier: Make atomic_notifiers use raw_spinlock

On Fri, Aug 06, 2021 at 08:06:53PM +0200, Sebastian Andrzej Siewior wrote:
> On 2021-08-06 20:02:42 [+0200], Peter Zijlstra wrote:
> > On Fri, Aug 06, 2021 at 04:07:18PM +0200, Sebastian Andrzej Siewior wrote:
> > > What do we do with this?
> > > Do we merge this as-is, add another "robust atomic notifier" using only
> > > raw_spinlock_t for registration and notification (for only
> > > cpu_pm_notifier_chain) instead of switching to raw_spinlock_t for all
> > > atomic notifier in -tree?
> > 
> > Right, so the problem I see with this is that
> > notifier_chain_{,un}register() are O(n). Hardly something we should be
> > putting under raw_spin_lock :/
> 
> Yup, pretty much. So we make one robust notifier for
> cpu_pm_notifier_chain?

Yeah, I suppose so :-( Ideally that whole pm notifier thing goes, but
that's *far* more work and I really don't want to be responsible for the
brain damange resulting from looking at all that 'special' idle code.