From 460822b0b1a77db859b0320469799fa4dbe4d367 Mon Sep 17 00:00:00 2001
From: =?utf8?q?Micha=C5=82=20Winiarski?= <michal.winiarski@intel.com>
Date: Tue, 3 Feb 2015 15:48:17 +0100
Subject: [PATCH] drm/i915: Prevent use-after-free in invalidate_range_start
 callback
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It's possible for invalidate_range_start mmu notifier callback to race
against userptr object release. If the gem object was released prior to
obtaining the spinlock in invalidate_range_start we're hitting null
pointer dereference.

Testcase: igt/gem_userptr_blits/stress-mm-invalidate-close
Testcase: igt/gem_userptr_blits/stress-mm-invalidate-close-overlap
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: stable@vger.kernel.org
[Jani: added code comment suggested by Chris]
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
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