Re: [Intel-gfx] [PATCH] drm/i915: Fix context IDs not released on driver hot unbind

[Chris Wilson <chris@chris-wilson.co.uk>]

Quoting Janusz Krzysztofik (2019-04-04 11:40:24)
> On Thu, 2019-04-04 at 11:28 +0100, Chris Wilson wrote:
> > Quoting Janusz Krzysztofik (2019-04-04 11:24:45)
> > > From: Janusz Krzysztofik <janusz.krzysztofik@intel.com>
> > > 
> > > In case the driver gets unbound while a device is open, kernel
> > > panic
> > > may be forced if a list of allocated context IDs is not empty.
> > > 
> > > When a device is open, the list may happen to be not empty because
> > > a
> > > context ID, once allocated by a context ID allocator to a context
> > > assosiated with that open file descriptor, is released as late as
> > > on device close.
> > > 
> > > On the other hand, there is a need to release all allocated context
> > > IDs
> > > and destroy the context ID allocator on driver unbind, even if a
> > > device
> > > is open, in order to free memory resources consumed and prevent
> > > from
> > > memory leaks.  The purpose of the forced kernel panic was to
> > > protect
> > > the context ID allocator from being silently destroyed if not all
> > > allocated IDs had been released.
> > 
> > Those open fd are still pointing into kernel memory where the driver
> > used to be. The panic is entirely correct, we should not be unloading
> > the module before those dangling pointers have been made safe.
> > 
> > This is papering over the symptom. How is the module being unloaded
> > with
> > open fd? 
> 
> A user can play with the driver unbind or device remove sysfs
> interface.

Sure, but we must still follow all the steps before _unloading_ the
module or else the user is left pointing into reused kernel memory.
-Chris