Messages in this thread | | | Date | Wed, 15 Jul 2015 18:20:02 +0200 | From | Takashi Iwai <> | Subject | Re: Is devm_* broken ? |
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On Wed, 15 Jul 2015 18:08:34 +0200, Laurent Pinchart wrote: > > Hello Takashi, > > On Wednesday 15 July 2015 17:51:28 Takashi Iwai wrote: > > On Wed, 15 Jul 2015 00:34:53 +0200, Laurent Pinchart wrote: > > > Hello, > > > > > > I came to realize not too long ago that the following sequence of events > > > will lead to a crash with any platform driver that uses devm_* and > > > creates device nodes. > > > > > > 1. Get a platform device bound it its driver > > > 2. Open the corresponding device node in userspace and keep it open > > > 3. Unbind the platform device from its driver through sysfs > > > > > > echo <device-name> > /sys/bus/platform/drivers/<driver-name>/unbind > > > > > > (or for hotpluggable devices just unplug the device) > > > > > > 4. Close the device node > > > 5. Enjoy the fireworks > > > > > > While having a device node open prevents modules from being unloaded, it > > > doesn't prevent devices from being unbound from drivers. If the driver > > > uses devm_* helpers to allocate memory the memory will be freed when the > > > device is unbound from the driver, but that memory will still be used by > > > any operation touching an open device node. > > > > > > Is devm_* inherently broken ? It's so widely used, tell me I'm missing > > > something obvious. > > > > I don't think this is specific to devm_*() but it's about the resource > > management in general. After bus or driver's remove callback, all > > device resources that have been assigned by the driver are supposed to > > be freed, or ready to be freed. > > The remove callback notifies drivers that the device has been removed and that > it's time to clean up. However, drivers have no control over userspace, so > they can't force applications to close all open file handles, unmap memory and > otherwise free all device-related resources immediately and synchronously. The > best a driver can do is prevent any new reference to a resource from being > taken by userspace (returning an error from open() for instance) and wait > until all existing references get released before finally freeing resources. > This is where devm_* hurts as a driver can't delay freeing resources until > after all references held by userspace are released.
Right, and this is what ALSA drivers does in general.
> If I were to switch the uvcvideo driver from kzalloc to devm_kzalloc it would > crash if the webcam gets disconnected while userspace has the V4L2 device node > open.
The disconnection is a bit different story, but I see your concern.
Takashi
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