On Tue, Feb 03, 2015 at 03:48:17PM +0100, Michał Winiarski wrote:
> 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*
> Cc: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Michał Winiarski <michal.winiarski@xxxxxxxxx>
Reviewed-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Though I would personally remove the extra newline and add an extra comment instead:
> ---
> drivers/gpu/drm/i915/i915_gem_userptr.c | 13 +++++++++++--
> 1 file changed, 11 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_gem_userptr.c b/drivers/gpu/drm/i915/i915_gem_userptr.c
> index d182058..64b8802 100644
> --- a/drivers/gpu/drm/i915/i915_gem_userptr.c
> +++ b/drivers/gpu/drm/i915/i915_gem_userptr.c
> @@ -113,7 +113,10 @@ restart:
> continue;
>
> obj = mo->obj;
> - drm_gem_object_reference(&obj->base);
> + if (!kref_get_unless_zero(&obj->base.refcount))
> + continue;
> +
> spin_unlock(&mn->lock);
>
> cancel_userptr(obj);
> @@ -149,7 +152,13 @@ static void i915_gem_userptr_mn_invalidate_range_start(struct mmu_notifier *_mn,
> it = interval_tree_iter_first(&mn->objects, start, end);
> if (it != NULL) {
> obj = container_of(it, struct i915_mmu_object, it)->obj;
> - drm_gem_object_reference(&obj->base);
/* The mmu_object is released late when
* destroying the GEM object so it is entirely
* possible to gain a reference on an object
* in the process of being freed since our
* serialisation is via the spinlock and not the
* struct_mutex - and consequently use it
* after it is freed and then double free it.
*/
> + if (!kref_get_unless_zero(&obj->base.refcount)) {
> + spin_unlock(&mn->lock);
> + serial = 0;
> + continue;
> + }
> +
> serial = mn->serial;
> }
> spin_unlock(&mn->lock);
-Chris
--
Chris Wilson, Intel Open Source Technology Centre
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