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[72.74.165.95]) by smtp.gmail.com with ESMTPSA id c21-v6sm522463qtj.22.2018.07.17.11.34.48 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Tue, 17 Jul 2018 11:34:48 -0700 (PDT) Message-ID: <2dbe75b1a83c025b9cddc229dbca9af6fb30111e.camel@redhat.com> Subject: Re: [Nouveau] [PATCH 1/5] drm/nouveau: Prevent RPM callback recursion in suspend/resume paths From: Lyude Paul Reply-To: lyude@redhat.com To: Lukas Wunner Cc: nouveau@lists.freedesktop.org, David Airlie , linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org, Ben Skeggs , linux-pm@vger.kernel.org Date: Tue, 17 Jul 2018 14:34:47 -0400 In-Reply-To: <20180717183211.GB18363@wunner.de> References: <20180716235936.11268-1-lyude@redhat.com> <20180716235936.11268-2-lyude@redhat.com> <20180717071641.GA5411@wunner.de> <20180717182041.GA18363@wunner.de> <20180717183211.GB18363@wunner.de> Organization: Red Hat Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.28.3 (3.28.3-1.fc28) Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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