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Date: Wed, 9 Nov 2016 18:27:44 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Andy Lutomirski <luto@amacapital.net>,
        Roman Pen <roman.penyaev@profitbricks.com>,
        Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@redhat.com>,
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        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Chunming Zhou <David1.Zhou@amd.com>,
        Alex Deucher <alexander.deucher@amd.com>
Subject: Re: [PATCH 2/2] kthread: don't use to_live_kthread() in
        kthread_park() and kthread_unpark()
Message-ID: <20161109172743.GB26446@redhat.com>
References: <20161025154301.GA12015@redhat.com> <CALCETrVmczve=knhYGrcXkeAXRQR_oY4ac2BSc7V0TEnx6-GFA@mail.gmail.com> <20161026141359.GA6893@redhat.com> <alpine.DEB.2.20.1610261650260.4983@nanos> <20161026155155.GA28832@redhat.com> <alpine.DEB.2.20.1610262027110.5013@nanos> <20161028161106.GA8933@redhat.com> <20161031200729.GA19430@redhat.com> <20161031200823.GC19430@redhat.com> <alpine.DEB.2.20.1611090908270.3501@nanos>
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On 11/09, Thomas Gleixner wrote:
>
> > - watchdog_park_threads() and it does not look nice. The code is actually
> >   correct, get_online_cpus() ensures that kthread_park() can't race with
> >   itself (note that kthread_park() can't handle this race correctly), but
> >   imo it should not use kthread_park() directly.
>
> Should we provide an interface through the smpboot thread infrastructure for
> this?

IMHO yes, I'll write another email.

> I can see why that gpu driver wants to use the park mechanism and I guess
> there are other legitimate use cases as well. I prefer to implement a
> park/unpark variant which is safe to use on arbitrary kthreads

Yes, agreed. Again, I'll write another email. Perhaps we should even keep
park/unpark exported and change them to avoid the races with exit/itself,
I dunno.

My real point was, imo the KTHREAD_IS_PER_CPU/__kthread_bind(kthread->cpu)
logic in kthread_unpark() should be private to smpboot.c/cpu.c.

I'll send another patch tomorrow. kthread_create_worker_on_cpu() ab-uses
this logic too for no reason, but this is trivial.

> Reviewed-by: Thomas Gleixner <tglx@linutronix.de>

Thanks!

Probably I should re-send these 2 short series to Ingo with your acks applied.

Oleg.