linux-kernel - Re: Is devm

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Date:	Wed, 15 Jul 2015 18:20:02 +0200
From:	Takashi Iwai <tiwai@...e.de>
To:	Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Is devm_* broken ?

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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