linux-kernel - Re: [PATCH] drm/i915: Preallocate mmu notifier to unbreak cpu hotplug deadlock

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Message-ID: <alpine.DEB.2.20.1710051816460.2083@nanos>
Date:   Thu, 5 Oct 2017 18:19:30 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Daniel Vetter <daniel@...ll.ch>
cc:     Daniel Vetter <daniel.vetter@...ll.ch>,
        Intel Graphics Development <intel-gfx@...ts.freedesktop.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        Tvrtko Ursulin <tvrtko.ursulin@...el.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Sasha Levin <alexander.levin@...izon.com>,
        Daniel Vetter <daniel.vetter@...el.com>
Subject: Re: [PATCH] drm/i915: Preallocate mmu notifier to unbreak cpu hotplug
 deadlock

On Thu, 5 Oct 2017, Daniel Vetter wrote:
> On Thu, Oct 05, 2017 at 05:23:20PM +0200, Thomas Gleixner wrote:
> > Aside of that, is it really required to use stomp_machine() for this
> > synchronization? We certainly have less intrusive mechansisms than that.
> 
> Yeah, the stop_machine needs to go, I'm working on something that uses
> rcu_read_lock+synchronize_rcu for this case. Probably shouldn't have
> merged even.
> 
> Now this one isn't the one I wanted to fix with this patch since there's
> clearly something dubious going on on the i915 side too.

I already wondered :)

> The proper trace, with the same part on the cpu hotplug side, highlights
> that you can't allocate a workqueue while hodling mmap_sem. That one
> matches patch description&diff a bit better :-)

> Sorry for misleading you, should have checked to attach the right one. No
> stop_machine()/i915_gem_set_wedged() in the below one.

Well the problem is more or less the same and what I said about solving it
in a different place is still valid. I think about it some more, but don't
expect wonders :)

Thanks,

	tglx