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

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Date:   Thu, 04 Apr 2019 11:28:59 +0100
From:   Chris Wilson <chris@...is-wilson.co.uk>
To:     Jani Nikula <jani.nikula@...ux.intel.com>,
        Janusz Krzysztofik <janusz.krzysztofik@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>
Cc:     Janusz Krzysztofik <janusz.krzysztofik@...el.com>,
        David Airlie <airlied@...ux.ie>,
        intel-gfx@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org
Subject: Re: [Intel-gfx] [PATCH] drm/i915: Fix context IDs not released on driver hot
 unbind

Quoting Janusz Krzysztofik (2019-04-04 11:24:45)
> From: Janusz Krzysztofik <janusz.krzysztofik@...el.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? 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