linux-kernel - Re: [PATCH 2/2] drm/i915: Use rcu instead of stop_machine in set

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Message-ID: <20171006111242.5vqwotkqxiaajsqh@hirez.programming.kicks-ass.net>
Date:   Fri, 6 Oct 2017 13:12:42 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Chris Wilson <chris@...is-wilson.co.uk>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Intel Graphics Development <intel-gfx@...ts.freedesktop.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Mika Kuoppala <mika.kuoppala@...el.com>,
        Marta Lofstedt <marta.lofstedt@...el.com>,
        Daniel Vetter <daniel.vetter@...el.com>
Subject: Re: [PATCH 2/2] drm/i915: Use rcu instead of stop_machine in
 set_wedged

On Fri, Oct 06, 2017 at 12:12:41PM +0200, Thomas Gleixner wrote:
> So if you don't have a compelling technical reason to
> use it, then it _is_ the wrong tool.

This. stop_machine() effectively takes down _all_ CPUs for the duration
of your callback. That is something you really should avoid at pretty
much any cost.