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Subject: Re: [Nouveau] [PATCH 1/5] drm/nouveau: Prevent RPM callback
 recursion in suspend/resume paths
From:   Lyude Paul <lyude@redhat.com>
Reply-To: lyude@redhat.com
To:     Lukas Wunner <lukas@wunner.de>
Cc:     nouveau@lists.freedesktop.org, David Airlie <airlied@linux.ie>,
        linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org,
        Ben Skeggs <bskeggs@redhat.com>, linux-pm@vger.kernel.org
Date:   Tue, 17 Jul 2018 14:34:47 -0400
In-Reply-To: <20180717183211.GB18363@wunner.de>
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         <20180717182041.GA18363@wunner.de>
         <cb5af1611622a080fdcbcf3bb794d8ebc39cae2a.camel@redhat.com>
         <20180717183211.GB18363@wunner.de>
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On Tue, 2018-07-17 at 20:32 +0200, Lukas Wunner wrote:
> On Tue, Jul 17, 2018 at 02:24:31PM -0400, Lyude Paul wrote:
> > On Tue, 2018-07-17 at 20:20 +0200, Lukas Wunner wrote:
> > > Okay, the PCI device is suspending and the nvkm_i2c_aux_acquire()
> > > wants it in resumed state, so is waiting forever for the device to
> > > runtime suspend in order to resume it again immediately afterwards.
> > > 
> > > The deadlock in the stack trace you've posted could be resolved using
> > > the technique I used in d61a5c106351 by adding the following to
> > > include/linux/pm_runtime.h:
> > > 
> > > static inline bool pm_runtime_status_suspending(struct device *dev)
> > > {
> > > 	return dev->power.runtime_status == RPM_SUSPENDING;
> > > }
> > > 
> > > static inline bool is_pm_work(struct device *dev)
> > > {
> > > 	struct work_struct *work = current_work();
> > > 
> > > 	return work && work->func == dev->power.work;
> > > }
> > > 
> > > Then adding this to nvkm_i2c_aux_acquire():
> > > 
> > > 	struct device *dev = pad->i2c->subdev.device->dev;
> > > 
> > > 	if (!(is_pm_work(dev) && pm_runtime_status_suspending(dev))) {
> > > 		ret = pm_runtime_get_sync(dev);
> > > 		if (ret < 0 && ret != -EACCES)
> > > 			return ret;
> > > 	}
> > > 
> > > But here's the catch:  This only works for an *async* runtime suspend.
> > > It doesn't work for pm_runtime_put_sync(), pm_runtime_suspend() etc,
> > > because then the runtime suspend is executed in the context of the caller,
> > > not in the context of dev->power.work.
> > > 
> > > So it's not a full solution, but hopefully something that gets you
> > > going.  I'm not really familiar with the code paths leading to
> > > nvkm_i2c_aux_acquire() to come up with a full solution off the top
> > > of my head I'm afraid.
> > 
> > OK-I was considering doing something similar to that commit beforehand but I
> > wasn't sure if I was going to just be hacking around an actual issue. That
> > doesn't seem to be the case. This is very helpful and hopefully I should be
> > able
> > to figure something out from this, thanks!
> 
> In some cases, the function acquiring the runtime PM ref is only called
> from a couple of places and then it would be feasible and appropriate
> to add a bool parameter to the function telling it to acquire the ref
> or not.  So the function is told using a parameter which context it's
> running in:  In the runtime_suspend code path or some other code path.
> 
> The technique to use current_work() is an alternative approach to figure
> out the context if passing in an additional parameter is not feasible
> for some reason.  That was the case with d61a5c106351.  That approach
> only works for work items though.

Something I'm curious about. This isn't the first time I've hit a situation like
this (see: the improper disable_depth fix I added into amdgpu I now need to go
and fix), which makes me wonder: is there actually any reason Linux's runtime PM
core doesn't just turn get/puts() in the context of s/r callbacks into no-ops by
default? 

> 
> Lukas