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

[Janusz Krzysztofik]

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 at 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.

Thanks,
Janusz

> If all the fd have been closed, how have we failed to flush and
> retire all requests (thereby unpinning the contexts and all other
> pointers).
> -Chris
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