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[2001:1c00:c1e:bf00:1054:9d19:e0f0:8214]) by smtp.gmail.com with ESMTPSA id y2sm1719269ejg.123.2021.04.29.05.30.42 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 29 Apr 2021 05:30:42 -0700 (PDT) Subject: Re: [PATCH 1/9] drm/connector: Make the drm_sysfs connector->kdev device hold a reference to the connector To: Daniel Vetter Cc: Maarten Lankhorst , Maxime Ripard , Thomas Zimmermann , David Airlie , Jani Nikula , Joonas Lahtinen , Rodrigo Vivi , Greg Kroah-Hartman , Guenter Roeck , Heikki Krogerus , intel-gfx , dri-devel@lists.freedesktop.org, platform-driver-x86@vger.kernel.org, linux-usb@vger.kernel.org References: <20210428215257.500088-1-hdegoede@redhat.com> <20210428215257.500088-2-hdegoede@redhat.com> From: Hans de Goede Message-ID: <1d6a190a-7492-0de3-2fce-595d3b710fdd@redhat.com> Date: Thu, 29 Apr 2021 14:30:41 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: platform-driver-x86@vger.kernel.org Hi, On 4/29/21 1:40 PM, Daniel Vetter wrote: > On Wed, Apr 28, 2021 at 11:52:49PM +0200, Hans de Goede wrote: >> Userspace could hold open a reference to the connector->kdev device, >> through e.g. holding a sysfs-atrtribute open after >> drm_sysfs_connector_remove() has been called. In this case the connector >> could be free-ed while the connector->kdev device's drvdata is still >> pointing to it. >> >> Give drm_connector devices there own device type, which allows >> us to specify our own release function and make drm_sysfs_connector_add() >> take a reference on the connector object, and have the new release >> function put the reference when the device is released. >> >> Giving drm_connector devices there own device type, will also allow >> checking if a device is a drm_connector device with a >> "if (device->type == &drm_sysfs_device_connector)" check. >> >> Note that the setting of the name member of the device_type struct will >> cause udev events for drm_connector-s to now contain DEVTYPE=drm_connector >> as extra info. So this extends the uevent part of the userspace API. >> >> Signed-off-by: Hans de Goede > > Are you sure? I thought sysfs is supposed to flush out any pending > operations (they complete fast) and handle open fd internally? So I did some digging in fs/kernfs and it looks like you right, once the file has been removed from sysfs any accesses through an open fd will fail with -ENODEV, interesting I did not know this. We still need this change though to make sure that the "drm/connector: Add drm_connector_find_by_fwnode() function" does not end up following a dangling drvdat pointer from one if the drm_connector kdev-s. The class_dev_iter_init() in drm_connector_find_by_fwnode() gets a reference on all devices and between getting that reference and it calling drm_connector_get() - drm_connector_unregister() may run and drop the possibly last reference to the drm_connector object, freeing it and leaving the kdev's drvdata as a dangling pointer. But I obviously need to rewrite the commit message of this commit as it currently is completely wrong. Maybe I should even squash this into the commit adding drm_connector_find_by_fwnode() ? Note sure about that though I personally think this is best kept as a new preparation patch but with a new commit msg. > Also I'd assume this creates a loop since the connector holds a reference > on the kdev? So I was wondering the same thing when working on this code and I checked. the reference on the kdev is dropped from: drm_connector_unregister() -> drm_sysfs_connector_remove() and then happens independent of the reference count on the connector-drm-obj dropping to 0. So what this change does is it keeps a reference to the drm_connector obj as long as someone is keeping a reference to the connnector->kdev device around after drm_connector_unregister() but as soon as that kdev device ref is dropped, so will the drm_connector's obj reference. I also tested this using a dock with DP MST, which dynamically adds/removes connectors on plug-in / plug-out of the dock-cable and I added a printk to the new drm_sysfs_connector_release() this adds and that printk triggered pretty much immediately on unplug as expected, releasing the extra drm_connector obj as soon as drm_connector_unregister() got called. Regards, Hans > -Daniel > >> --- >> drivers/gpu/drm/drm_sysfs.c | 54 +++++++++++++++++++++++++++++++------ >> 1 file changed, 46 insertions(+), 8 deletions(-) >> >> diff --git a/drivers/gpu/drm/drm_sysfs.c b/drivers/gpu/drm/drm_sysfs.c >> index f0336c804639..c344c6d5e738 100644 >> --- a/drivers/gpu/drm/drm_sysfs.c >> +++ b/drivers/gpu/drm/drm_sysfs.c >> @@ -50,6 +50,10 @@ static struct device_type drm_sysfs_device_minor = { >> .name = "drm_minor" >> }; >> >> +static struct device_type drm_sysfs_device_connector = { >> + .name = "drm_connector", >> +}; >> + >> struct class *drm_class; >> >> static char *drm_devnode(struct device *dev, umode_t *mode) >> @@ -271,30 +275,64 @@ static const struct attribute_group *connector_dev_groups[] = { >> NULL >> }; >> >> +static void drm_sysfs_connector_release(struct device *dev) >> +{ >> + struct drm_connector *connector = to_drm_connector(dev); >> + >> + drm_connector_put(connector); >> + kfree(dev); >> +} >> + >> int drm_sysfs_connector_add(struct drm_connector *connector) >> { >> struct drm_device *dev = connector->dev; >> + struct device *kdev; >> + int r; >> >> if (connector->kdev) >> return 0; >> >> - connector->kdev = >> - device_create_with_groups(drm_class, dev->primary->kdev, 0, >> - connector, connector_dev_groups, >> - "card%d-%s", dev->primary->index, >> - connector->name); >> + kdev = kzalloc(sizeof(*kdev), GFP_KERNEL); >> + if (!kdev) >> + return -ENOMEM; >> + >> + device_initialize(kdev); >> + kdev->class = drm_class; >> + kdev->type = &drm_sysfs_device_connector; >> + kdev->parent = dev->primary->kdev; >> + kdev->groups = connector_dev_groups; >> + kdev->release = drm_sysfs_connector_release; >> + dev_set_drvdata(kdev, connector); >> + >> + r = dev_set_name(kdev, "card%d-%s", dev->primary->index, connector->name); >> + if (r) >> + goto err_free; >> + >> DRM_DEBUG("adding \"%s\" to sysfs\n", >> connector->name); >> >> - if (IS_ERR(connector->kdev)) { >> - DRM_ERROR("failed to register connector device: %ld\n", PTR_ERR(connector->kdev)); >> - return PTR_ERR(connector->kdev); >> + r = device_add(kdev); >> + if (r) { >> + DRM_ERROR("failed to register connector device: %d\n", r); >> + goto err_free; >> } >> >> + /* >> + * Ensure the connector object does not get free-ed if userspace still has >> + * references open to the device through e.g. the connector sysfs-attributes. >> + */ >> + drm_connector_get(connector); >> + >> + connector->kdev = kdev; >> + >> if (connector->ddc) >> return sysfs_create_link(&connector->kdev->kobj, >> &connector->ddc->dev.kobj, "ddc"); >> return 0; >> + >> +err_free: >> + put_device(kdev); >> + return r; >> } >> >> void drm_sysfs_connector_remove(struct drm_connector *connector) >> -- >> 2.31.1 >> >