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Date: Thu, 5 Oct 2017 18:19:30 +0200 (CEST)
From: Thomas Gleixner <tglx@linutronix.de>
To: Daniel Vetter <daniel@ffwll.ch>
cc: Daniel Vetter <daniel.vetter@ffwll.ch>,
        Intel Graphics Development <intel-gfx@lists.freedesktop.org>,
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        Peter Zijlstra <peterz@infradead.org>,
        Sasha Levin <alexander.levin@verizon.com>,
        Daniel Vetter <daniel.vetter@intel.com>
Subject: Re: [PATCH] drm/i915: Preallocate mmu notifier to unbreak cpu hotplug
 deadlock
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References: <20171005132206.21365-1-daniel.vetter@ffwll.ch> <alpine.DEB.2.20.1710051614050.2083@nanos> <20171005155848.tqiveborg5aqc4ep@phenom.ffwll.local>
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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