[v3,2/3] drm/i915: Fix userptr deadlock with aliased GTT mmappings
diff mbox

Message ID 1439196700-20045-2-git-send-email-chris@chris-wilson.co.uk
State New
Headers show

Commit Message

Chris Wilson Aug. 10, 2015, 8:51 a.m. UTC
Micha? Winiarski found a really evil way to trigger a struct_mutex
deadlock with userptr. He found that if he allocated a userptr bo and
then GTT mmaped another bo, or even itself, at the same address as the
userptr using MAP_FIXED, he could then cause a deadlock any time we then
had to invalidate the GTT mmappings (so at will). Tvrtko then found by
repeatedly allocating GTT mmappings he could alias with an old userptr
mmap and also trigger the deadlock.

To counter act the deadlock, we make the observation that we only need
to take the struct_mutex if the object has any pages to revoke, and that
before userspace can alias with the userptr address space, it must have
invalidated the userptr->pages. Thus if we can check for those pages
outside of the struct_mutex, we can avoid the deadlock. To do so we
introduce a separate flag for userptr objects that we can inspect from
the mmu-notifier underneath its spinlock.

The patch makes one eye-catching change. That is the removal serial=0
after detecting a to-be-freed object inside the invalidate walker. I
felt setting serial=0 was a questionable pessimisation: it denies us the
chance to reuse the current iterator for the next loop (before it is
freed) and being explicit makes the reader question the validity of the
locking (since the object-free race could occur elsewhere). The
serialisation of the iterator is through the spinlock, if the object is
freed before the next loop then the notifier.serial will be incremented
and we start the walk from the beginning as we detect the invalid cache.

To try and tame the error paths and interactions with the userptr->active
flag, we have to do a fair amount of rearranging of get_pages_userptr().

v2: Grammar fixes
v3: Reorder set-active so that it is only set when obj->pages is set
(and so needs cancellation). Only the order of setting obj->pages and
the active-flag is crucial. Calling gup after invalidate-range begin
means the userptr sees the new set of backing storage (and so will not
need to invalidate its new pages), but we have to be careful not to set
the active-flag prior to successfully establishing obj->pages.
v4: Take the active->flag early so we know in the mmu-notifier when we
have to cancel a pending gup-worker.

Reported-by: Micha? Winiarski <michal.winiarski@intel.com>
Testcase: igt/gem_userptr_blits/map-fixed*
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Micha? Winiarski <michal.winiarski@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: stable@vger.kernel.org
---
 drivers/gpu/drm/i915/i915_gem_userptr.c | 111 ++++++++++++++++++++++----------
 1 file changed, 78 insertions(+), 33 deletions(-)

Comments

Tvrtko Ursulin Sept. 9, 2015, 1:56 p.m. UTC | #1
Hi,

On 08/10/2015 09:51 AM, Chris Wilson wrote:
> Micha? Winiarski found a really evil way to trigger a struct_mutex
> deadlock with userptr. He found that if he allocated a userptr bo and
> then GTT mmaped another bo, or even itself, at the same address as the
> userptr using MAP_FIXED, he could then cause a deadlock any time we then
> had to invalidate the GTT mmappings (so at will). Tvrtko then found by
> repeatedly allocating GTT mmappings he could alias with an old userptr
> mmap and also trigger the deadlock.
> 
> To counter act the deadlock, we make the observation that we only need
> to take the struct_mutex if the object has any pages to revoke, and that
> before userspace can alias with the userptr address space, it must have
> invalidated the userptr->pages. Thus if we can check for those pages
> outside of the struct_mutex, we can avoid the deadlock. To do so we
> introduce a separate flag for userptr objects that we can inspect from
> the mmu-notifier underneath its spinlock.
> 
> The patch makes one eye-catching change. That is the removal serial=0
> after detecting a to-be-freed object inside the invalidate walker. I
> felt setting serial=0 was a questionable pessimisation: it denies us the
> chance to reuse the current iterator for the next loop (before it is
> freed) and being explicit makes the reader question the validity of the
> locking (since the object-free race could occur elsewhere). The
> serialisation of the iterator is through the spinlock, if the object is
> freed before the next loop then the notifier.serial will be incremented
> and we start the walk from the beginning as we detect the invalid cache.
> 
> To try and tame the error paths and interactions with the userptr->active
> flag, we have to do a fair amount of rearranging of get_pages_userptr().
> 
> v2: Grammar fixes
> v3: Reorder set-active so that it is only set when obj->pages is set
> (and so needs cancellation). Only the order of setting obj->pages and
> the active-flag is crucial. Calling gup after invalidate-range begin
> means the userptr sees the new set of backing storage (and so will not
> need to invalidate its new pages), but we have to be careful not to set
> the active-flag prior to successfully establishing obj->pages.
> v4: Take the active->flag early so we know in the mmu-notifier when we
> have to cancel a pending gup-worker.
> 
> Reported-by: Micha? Winiarski <michal.winiarski@intel.com>
> Testcase: igt/gem_userptr_blits/map-fixed*
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Micha? Winiarski <michal.winiarski@intel.com>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> Cc: stable@vger.kernel.org
> ---
>   drivers/gpu/drm/i915/i915_gem_userptr.c | 111 ++++++++++++++++++++++----------
>   1 file changed, 78 insertions(+), 33 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_gem_userptr.c b/drivers/gpu/drm/i915/i915_gem_userptr.c
> index 800a5394aa1e..e21f885db87b 100644
> --- a/drivers/gpu/drm/i915/i915_gem_userptr.c
> +++ b/drivers/gpu/drm/i915/i915_gem_userptr.c
> @@ -59,6 +59,7 @@ struct i915_mmu_object {
>   	struct interval_tree_node it;
>   	struct list_head link;
>   	struct drm_i915_gem_object *obj;
> +	bool active;
>   	bool is_linear;
>   };
>   
> @@ -114,7 +115,8 @@ restart:
>   
>   		obj = mo->obj;
>   
> -		if (!kref_get_unless_zero(&obj->base.refcount))
> +		if (!mo->active ||
> +		    !kref_get_unless_zero(&obj->base.refcount))
>   			continue;
>   
>   		spin_unlock(&mn->lock);
> @@ -151,7 +153,8 @@ static void i915_gem_userptr_mn_invalidate_range_start(struct mmu_notifier *_mn,
>   		else
>   			it = interval_tree_iter_first(&mn->objects, start, end);
>   		if (it != NULL) {
> -			obj = container_of(it, struct i915_mmu_object, it)->obj;
> +			struct i915_mmu_object *mo =
> +				container_of(it, struct i915_mmu_object, it);
>   
>   			/* The mmu_object is released late when destroying the
>   			 * GEM object so it is entirely possible to gain a
> @@ -160,11 +163,9 @@ static void i915_gem_userptr_mn_invalidate_range_start(struct mmu_notifier *_mn,
>   			 * 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;
> -			}
> +			if (mo->active &&
> +			    kref_get_unless_zero(&mo->obj->base.refcount))
> +				obj = mo->obj;
>   
>   			serial = mn->serial;
>   		}
> @@ -566,6 +567,30 @@ __i915_gem_userptr_set_pages(struct drm_i915_gem_object *obj,
>   }
>   
>   static void
> +__i915_gem_userptr_set_active(struct drm_i915_gem_object *obj,
> +			      bool value)
> +{
> +	/* During mm_invalidate_range we need to cancel any userptr that
> +	 * overlaps the range being invalidated. Doing so requires the
> +	 * struct_mutex, and that risks recursion. In order to cause
> +	 * recursion, the user must alias the userptr address space with
> +	 * a GTT mmapping (possible with a MAP_FIXED) - then when we have
> +	 * to invalidate that mmaping, mm_invalidate_range is called with
> +	 * the userptr address *and* the struct_mutex held.  To prevent that
> +	 * we set a flag under the i915_mmu_notifier spinlock to indicate
> +	 * whether this object is valid.
> +	 */
> +#if defined(CONFIG_MMU_NOTIFIER)
> +	if (obj->userptr.mmu_object == NULL)
> +		return;
> +
> +	spin_lock(&obj->userptr.mmu_object->mn->lock);
> +	obj->userptr.mmu_object->active = value;
> +	spin_unlock(&obj->userptr.mmu_object->mn->lock);
> +#endif
> +}
> +
> +static void
>   __i915_gem_userptr_get_pages_worker(struct work_struct *_work)
>   {
>   	struct get_pages_work *work = container_of(_work, typeof(*work), work);
> @@ -613,6 +638,8 @@ __i915_gem_userptr_get_pages_worker(struct work_struct *_work)
>   			}
>   		}
>   		obj->userptr.work = ERR_PTR(ret);
> +		if (ret)
> +			__i915_gem_userptr_set_active(obj, false);
>   	}
>   
>   	obj->userptr.workers--;
> @@ -649,6 +676,18 @@ i915_gem_userptr_get_pages(struct drm_i915_gem_object *obj)
>   	 * to the vma (discard or cloning) which should prevent the more
>   	 * egregious cases from causing harm.
>   	 */
> +	if (IS_ERR(obj->userptr.work)) {
> +		/* active flag will have been dropped already by the worker */
> +		ret = PTR_ERR(obj->userptr.work);
> +		obj->userptr.work = NULL;
> +		return ret;
> +	}
> +	if (obj->userptr.work)
> +		/* active flag should still be held for the pending work */
> +		return -EAGAIN;
> +
> +	/* Let the mmu-notifier know that we have begun and need cancellation */
> +	__i915_gem_userptr_set_active(obj, true);
>   
>   	pvec = NULL;
>   	pinned = 0;
> @@ -657,8 +696,10 @@ i915_gem_userptr_get_pages(struct drm_i915_gem_object *obj)
>   			       GFP_TEMPORARY | __GFP_NOWARN | __GFP_NORETRY);
>   		if (pvec == NULL) {
>   			pvec = drm_malloc_ab(num_pages, sizeof(struct page *));
> -			if (pvec == NULL)
> -				return -ENOMEM;
> +			if (pvec == NULL) {
> +				ret = -ENOMEM;
> +				goto err;
> +			}
>   		}
>   
>   		pinned = __get_user_pages_fast(obj->userptr.ptr, num_pages,
> @@ -667,7 +708,7 @@ i915_gem_userptr_get_pages(struct drm_i915_gem_object *obj)
>   	if (pinned < num_pages) {
>   		if (pinned < 0) {
>   			ret = pinned;
> -			pinned = 0;
> +			goto err;
>   		} else {
>   			/* Spawn a worker so that we can acquire the
>   			 * user pages without holding our mutex. Access
> @@ -688,44 +729,47 @@ i915_gem_userptr_get_pages(struct drm_i915_gem_object *obj)
>   			 * that error back to this function through
>   			 * obj->userptr.work = ERR_PTR.
>   			 */
> +
>   			ret = -EAGAIN;
> -			if (obj->userptr.work == NULL &&
> -			    obj->userptr.workers < I915_GEM_USERPTR_MAX_WORKERS) {
> +			if (obj->userptr.workers < I915_GEM_USERPTR_MAX_WORKERS) {
>   				struct get_pages_work *work;
>   
>   				work = kmalloc(sizeof(*work), GFP_KERNEL);
> -				if (work != NULL) {
> -					obj->userptr.work = &work->work;
> -					obj->userptr.workers++;
> +				if (work == NULL) {
> +					ret = -ENOMEM;
> +					goto err;
> +				}
> +				obj->userptr.work = &work->work;
> +				obj->userptr.workers++;
>   
> -					work->obj = obj;
> -					drm_gem_object_reference(&obj->base);
> +				work->obj = obj;
> +				drm_gem_object_reference(&obj->base);
>   
> -					work->task = current;
> -					get_task_struct(work->task);
> +				work->task = current;
> +				get_task_struct(work->task);
>   
> -					INIT_WORK(&work->work, __i915_gem_userptr_get_pages_worker);
> -					schedule_work(&work->work);
> -				} else
> -					ret = -ENOMEM;
> -			} else {
> -				if (IS_ERR(obj->userptr.work)) {
> -					ret = PTR_ERR(obj->userptr.work);
> -					obj->userptr.work = NULL;
> -				}
> +				INIT_WORK(&work->work,
> +					  __i915_gem_userptr_get_pages_worker);
> +				schedule_work(&work->work);
>   			}
> +			goto err_active;
>   		}
>   	} else {
>   		ret = __i915_gem_userptr_set_pages(obj, pvec, num_pages);
> -		if (ret == 0) {
> -			obj->userptr.work = NULL;
> -			pinned = 0;
> -		}
> +		if (ret)
> +			goto err;
>   	}
>   
> -	release_pages(pvec, pinned, 0);
> +out:
>   	drm_free_large(pvec);
>   	return ret;
> +
> +err:
> +	/* No pages here, no need for the mmu-notifier to wake us */
> +	__i915_gem_userptr_set_active(obj, false);
> +err_active:
> +	release_pages(pvec, pinned, 0);
> +	goto out;
>   }

I don't like the goto dance. Would something like the below be clearer?

========================================================================
static int
i915_gem_userptr_get_pages(struct drm_i915_gem_object *obj)
{
	const int num_pages = obj->base.size >> PAGE_SHIFT;
	struct page **pvec;
	int pinned, ret;

	/* If userspace should engineer that these pages are replaced in
	 * the vma between us binding this page into the GTT and completion
	 * of rendering... Their loss. If they change the mapping of their
	 * pages they need to create a new bo to point to the new vma.
	 *
	 * However, that still leaves open the possibility of the vma
	 * being copied upon fork. Which falls under the same userspace
	 * synchronisation issue as a regular bo, except that this time
	 * the process may not be expecting that a particular piece of
	 * memory is tied to the GPU.
	 *
	 * Fortunately, we can hook into the mmu_notifier in order to
	 * discard the page references prior to anything nasty happening
	 * to the vma (discard or cloning) which should prevent the more
	 * egregious cases from causing harm.
	 */
	if (IS_ERR(obj->userptr.work)) {
		/* active flag will have been dropped already by the worker */
		ret = PTR_ERR(obj->userptr.work);
		obj->userptr.work = NULL;
		return ret;
	}
	if (obj->userptr.work)
		/* active flag should still be held for the pending work */
		return -EAGAIN;

	/* Let the mmu-notifier know that we have begun and need cancellation */
	__i915_gem_userptr_set_active(obj, true);

	pvec = NULL;
	pinned = 0;
	if (obj->userptr.mm->mm == current->mm) {
		pvec = kmalloc(num_pages*sizeof(struct page *),
			       GFP_TEMPORARY | __GFP_NOWARN | __GFP_NORETRY);
		if (pvec == NULL) {
			pvec = drm_malloc_ab(num_pages, sizeof(struct page *));
			if (pvec == NULL) {
				ret = -ENOMEM;
				goto err_pvec;
			}
		}

		pinned = __get_user_pages_fast(obj->userptr.ptr, num_pages,
					       !obj->userptr.read_only, pvec);
	}

	if (pinned == num_pages) {
		ret = __i915_gem_userptr_set_pages(obj, pvec, num_pages);
		if (ret)
			release_pages(pvec, pinned, 0);
		goto out_gup;
	} else if (pinned < 0)
		ret = pinned;
		goto out_gup;
	}

	/* Spawn a worker so that we can acquire the
	 * user pages without holding our mutex. Access
	 * to the user pages requires mmap_sem, and we have
	 * a strict lock ordering of mmap_sem, struct_mutex -
	 * we already hold struct_mutex here and so cannot
	 * call gup without encountering a lock inversion.
	 *
	 * Userspace will keep on repeating the operation
	 * (thanks to EAGAIN) until either we hit the fast
	 * path or the worker completes. If the worker is
	 * cancelled or superseded, the task is still run
	 * but the results ignored. (This leads to
	 * complications that we may have a stray object
	 * refcount that we need to be wary of when
	 * checking for existing objects during creation.)
	 * If the worker encounters an error, it reports
	 * that error back to this function through
	 * obj->userptr.work = ERR_PTR.
	 */

	if (pvec) {
		release_pages(pvec, pinned, 0);
		drm_free_large(pvec);
	}
	
	ret = -EAGAIN;
	if (obj->userptr.workers < I915_GEM_USERPTR_MAX_WORKERS) {
		struct get_pages_work *work;

		work = kmalloc(sizeof(*work), GFP_KERNEL);
		if (work == NULL) {
			ret = -ENOMEM;
			goto err_pvec;
		}
		obj->userptr.work = &work->work;
		obj->userptr.workers++;

		work->obj = obj;
		drm_gem_object_reference(&obj->base);

		work->task = current;
		get_task_struct(work->task);

		INIT_WORK(&work->work,
				__i915_gem_userptr_get_pages_worker);
		schedule_work(&work->work);
	}

	return ret;

out_gup:
	drm_free_large(pvec);
err_pvec:
	/* All done, no need for the mmu-notifier to wake us */
	__i915_gem_userptr_set_active(obj, false);

	return ret;
}
======================================================================

I think it is correct but please double check me. Maybe needs more comments
for the main three options (gup ok, gup fail, gup incomplete) but I think it
is worth if for nothing for avoidance of goto up-down jumps.

Regards,

Tvrtko
Chris Wilson Sept. 9, 2015, 3:03 p.m. UTC | #2
On Wed, Sep 09, 2015 at 02:56:16PM +0100, Tvrtko Ursulin wrote:
> 
> Hi,
> 
> On 08/10/2015 09:51 AM, Chris Wilson wrote:
> > +out:
> >   	drm_free_large(pvec);
> >   	return ret;
> > +
> > +err:
> > +	/* No pages here, no need for the mmu-notifier to wake us */
> > +	__i915_gem_userptr_set_active(obj, false);
> > +err_active:
> > +	release_pages(pvec, pinned, 0);
> > +	goto out;
> >   }
> 
> I don't like the goto dance. Would something like the below be clearer?

We can condense it if we use a bool active and then feed everything
through the single exit path:

	active = false;
        if (pinned < 0)
                ret = pinned, pinned = 0;
        else if (pinned < num_pages)
                ret = __i915_gem_userptr_get_pages_queue(obj, &active);
        else
                ret = __i915_gem_userptr_set_pages(obj, pvec, num_pages);
        if (ret) {
                __i915_gem_userptr_set_active(obj, active);
                release_pages(pvec, pinned, 0);
        }
        drm_free_large(pvec);
        return ret;

Not happy with _queue. I guess i915_gem_userptr_get_pages_via_worker()
is better. Or i915_gem_userptr_get_pages_deferred().
-Chris
Tvrtko Ursulin Sept. 10, 2015, 9:44 a.m. UTC | #3
On 09/09/2015 04:03 PM, Chris Wilson wrote:
> On Wed, Sep 09, 2015 at 02:56:16PM +0100, Tvrtko Ursulin wrote:
>>
>> Hi,
>>
>> On 08/10/2015 09:51 AM, Chris Wilson wrote:
>>> +out:
>>>    	drm_free_large(pvec);
>>>    	return ret;
>>> +
>>> +err:
>>> +	/* No pages here, no need for the mmu-notifier to wake us */
>>> +	__i915_gem_userptr_set_active(obj, false);
>>> +err_active:
>>> +	release_pages(pvec, pinned, 0);
>>> +	goto out;
>>>    }
>>
>> I don't like the goto dance. Would something like the below be clearer?
>
> We can condense it if we use a bool active and then feed everything
> through the single exit path:
>
> 	active = false;
>          if (pinned < 0)
>                  ret = pinned, pinned = 0;
>          else if (pinned < num_pages)
>                  ret = __i915_gem_userptr_get_pages_queue(obj, &active);
>          else
>                  ret = __i915_gem_userptr_set_pages(obj, pvec, num_pages);
>          if (ret) {
>                  __i915_gem_userptr_set_active(obj, active);
>                  release_pages(pvec, pinned, 0);
>          }
>          drm_free_large(pvec);
>          return ret;
>
> Not happy with _queue. I guess i915_gem_userptr_get_pages_via_worker()
> is better. Or i915_gem_userptr_get_pages_deferred().

Looks much better on a glance. If release_pages with pinned = 0 is OK.

For the queueue/via_worker/deferred maybe _schedule_get_pages_worker?

Tvrtko
Chris Wilson Sept. 10, 2015, 9:51 a.m. UTC | #4
On Thu, Sep 10, 2015 at 10:44:14AM +0100, Tvrtko Ursulin wrote:
> 
> On 09/09/2015 04:03 PM, Chris Wilson wrote:
> >On Wed, Sep 09, 2015 at 02:56:16PM +0100, Tvrtko Ursulin wrote:
> >>
> >>Hi,
> >>
> >>On 08/10/2015 09:51 AM, Chris Wilson wrote:
> >>>+out:
> >>>   	drm_free_large(pvec);
> >>>   	return ret;
> >>>+
> >>>+err:
> >>>+	/* No pages here, no need for the mmu-notifier to wake us */
> >>>+	__i915_gem_userptr_set_active(obj, false);
> >>>+err_active:
> >>>+	release_pages(pvec, pinned, 0);
> >>>+	goto out;
> >>>   }
> >>
> >>I don't like the goto dance. Would something like the below be clearer?
> >
> >We can condense it if we use a bool active and then feed everything
> >through the single exit path:
> >
> >	active = false;
> >         if (pinned < 0)
> >                 ret = pinned, pinned = 0;
> >         else if (pinned < num_pages)
> >                 ret = __i915_gem_userptr_get_pages_queue(obj, &active);
> >         else
> >                 ret = __i915_gem_userptr_set_pages(obj, pvec, num_pages);
> >         if (ret) {
> >                 __i915_gem_userptr_set_active(obj, active);
> >                 release_pages(pvec, pinned, 0);
> >         }
> >         drm_free_large(pvec);
> >         return ret;
> >
> >Not happy with _queue. I guess i915_gem_userptr_get_pages_via_worker()
> >is better. Or i915_gem_userptr_get_pages_deferred().
> 
> Looks much better on a glance. If release_pages with pinned = 0 is OK.

It does a loop over for(int i = 0; i < pinned; i++); so it is fine as it
was.

> For the queueue/via_worker/deferred maybe _schedule_get_pages_worker?

I want to keep the i915_gem_object_get_pages prefix (haven't yet broken
that pattern, so no reason to start now), but _schedule seems
reasonable.
-Chris

Patch
diff mbox

diff --git a/drivers/gpu/drm/i915/i915_gem_userptr.c b/drivers/gpu/drm/i915/i915_gem_userptr.c
index 800a5394aa1e..e21f885db87b 100644
--- a/drivers/gpu/drm/i915/i915_gem_userptr.c
+++ b/drivers/gpu/drm/i915/i915_gem_userptr.c
@@ -59,6 +59,7 @@  struct i915_mmu_object {
 	struct interval_tree_node it;
 	struct list_head link;
 	struct drm_i915_gem_object *obj;
+	bool active;
 	bool is_linear;
 };
 
@@ -114,7 +115,8 @@  restart:
 
 		obj = mo->obj;
 
-		if (!kref_get_unless_zero(&obj->base.refcount))
+		if (!mo->active ||
+		    !kref_get_unless_zero(&obj->base.refcount))
 			continue;
 
 		spin_unlock(&mn->lock);
@@ -151,7 +153,8 @@  static void i915_gem_userptr_mn_invalidate_range_start(struct mmu_notifier *_mn,
 		else
 			it = interval_tree_iter_first(&mn->objects, start, end);
 		if (it != NULL) {
-			obj = container_of(it, struct i915_mmu_object, it)->obj;
+			struct i915_mmu_object *mo =
+				container_of(it, struct i915_mmu_object, it);
 
 			/* The mmu_object is released late when destroying the
 			 * GEM object so it is entirely possible to gain a
@@ -160,11 +163,9 @@  static void i915_gem_userptr_mn_invalidate_range_start(struct mmu_notifier *_mn,
 			 * 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;
-			}
+			if (mo->active &&
+			    kref_get_unless_zero(&mo->obj->base.refcount))
+				obj = mo->obj;
 
 			serial = mn->serial;
 		}
@@ -566,6 +567,30 @@  __i915_gem_userptr_set_pages(struct drm_i915_gem_object *obj,
 }
 
 static void
+__i915_gem_userptr_set_active(struct drm_i915_gem_object *obj,
+			      bool value)
+{
+	/* During mm_invalidate_range we need to cancel any userptr that
+	 * overlaps the range being invalidated. Doing so requires the
+	 * struct_mutex, and that risks recursion. In order to cause
+	 * recursion, the user must alias the userptr address space with
+	 * a GTT mmapping (possible with a MAP_FIXED) - then when we have
+	 * to invalidate that mmaping, mm_invalidate_range is called with
+	 * the userptr address *and* the struct_mutex held.  To prevent that
+	 * we set a flag under the i915_mmu_notifier spinlock to indicate
+	 * whether this object is valid.
+	 */
+#if defined(CONFIG_MMU_NOTIFIER)
+	if (obj->userptr.mmu_object == NULL)
+		return;
+
+	spin_lock(&obj->userptr.mmu_object->mn->lock);
+	obj->userptr.mmu_object->active = value;
+	spin_unlock(&obj->userptr.mmu_object->mn->lock);
+#endif
+}
+
+static void
 __i915_gem_userptr_get_pages_worker(struct work_struct *_work)
 {
 	struct get_pages_work *work = container_of(_work, typeof(*work), work);
@@ -613,6 +638,8 @@  __i915_gem_userptr_get_pages_worker(struct work_struct *_work)
 			}
 		}
 		obj->userptr.work = ERR_PTR(ret);
+		if (ret)
+			__i915_gem_userptr_set_active(obj, false);
 	}
 
 	obj->userptr.workers--;
@@ -649,6 +676,18 @@  i915_gem_userptr_get_pages(struct drm_i915_gem_object *obj)
 	 * to the vma (discard or cloning) which should prevent the more
 	 * egregious cases from causing harm.
 	 */
+	if (IS_ERR(obj->userptr.work)) {
+		/* active flag will have been dropped already by the worker */
+		ret = PTR_ERR(obj->userptr.work);
+		obj->userptr.work = NULL;
+		return ret;
+	}
+	if (obj->userptr.work)
+		/* active flag should still be held for the pending work */
+		return -EAGAIN;
+
+	/* Let the mmu-notifier know that we have begun and need cancellation */
+	__i915_gem_userptr_set_active(obj, true);
 
 	pvec = NULL;
 	pinned = 0;
@@ -657,8 +696,10 @@  i915_gem_userptr_get_pages(struct drm_i915_gem_object *obj)
 			       GFP_TEMPORARY | __GFP_NOWARN | __GFP_NORETRY);
 		if (pvec == NULL) {
 			pvec = drm_malloc_ab(num_pages, sizeof(struct page *));
-			if (pvec == NULL)
-				return -ENOMEM;
+			if (pvec == NULL) {
+				ret = -ENOMEM;
+				goto err;
+			}
 		}
 
 		pinned = __get_user_pages_fast(obj->userptr.ptr, num_pages,
@@ -667,7 +708,7 @@  i915_gem_userptr_get_pages(struct drm_i915_gem_object *obj)
 	if (pinned < num_pages) {
 		if (pinned < 0) {
 			ret = pinned;
-			pinned = 0;
+			goto err;
 		} else {
 			/* Spawn a worker so that we can acquire the
 			 * user pages without holding our mutex. Access
@@ -688,44 +729,47 @@  i915_gem_userptr_get_pages(struct drm_i915_gem_object *obj)
 			 * that error back to this function through
 			 * obj->userptr.work = ERR_PTR.
 			 */
+
 			ret = -EAGAIN;
-			if (obj->userptr.work == NULL &&
-			    obj->userptr.workers < I915_GEM_USERPTR_MAX_WORKERS) {
+			if (obj->userptr.workers < I915_GEM_USERPTR_MAX_WORKERS) {
 				struct get_pages_work *work;
 
 				work = kmalloc(sizeof(*work), GFP_KERNEL);
-				if (work != NULL) {
-					obj->userptr.work = &work->work;
-					obj->userptr.workers++;
+				if (work == NULL) {
+					ret = -ENOMEM;
+					goto err;
+				}
+				obj->userptr.work = &work->work;
+				obj->userptr.workers++;
 
-					work->obj = obj;
-					drm_gem_object_reference(&obj->base);
+				work->obj = obj;
+				drm_gem_object_reference(&obj->base);
 
-					work->task = current;
-					get_task_struct(work->task);
+				work->task = current;
+				get_task_struct(work->task);
 
-					INIT_WORK(&work->work, __i915_gem_userptr_get_pages_worker);
-					schedule_work(&work->work);
-				} else
-					ret = -ENOMEM;
-			} else {
-				if (IS_ERR(obj->userptr.work)) {
-					ret = PTR_ERR(obj->userptr.work);
-					obj->userptr.work = NULL;
-				}
+				INIT_WORK(&work->work,
+					  __i915_gem_userptr_get_pages_worker);
+				schedule_work(&work->work);
 			}
+			goto err_active;
 		}
 	} else {
 		ret = __i915_gem_userptr_set_pages(obj, pvec, num_pages);
-		if (ret == 0) {
-			obj->userptr.work = NULL;
-			pinned = 0;
-		}
+		if (ret)
+			goto err;
 	}
 
-	release_pages(pvec, pinned, 0);
+out:
 	drm_free_large(pvec);
 	return ret;
+
+err:
+	/* No pages here, no need for the mmu-notifier to wake us */
+	__i915_gem_userptr_set_active(obj, false);
+err_active:
+	release_pages(pvec, pinned, 0);
+	goto out;
 }
 
 static void
@@ -734,6 +778,7 @@  i915_gem_userptr_put_pages(struct drm_i915_gem_object *obj)
 	struct sg_page_iter sg_iter;
 
 	BUG_ON(obj->userptr.work != NULL);
+	__i915_gem_userptr_set_active(obj, false);
 
 	if (obj->madv != I915_MADV_WILLNEED)
 		obj->dirty = 0;