[PATCH v4 1/3] drm/i915: Only update the current userptr worker

[PATCH v4 1/3] drm/i915: Only update the current userptr worker

[Chris Wilson]

The userptr worker allows for a slight race condition where upon there
may two or more threads calling get_user_pages for the same object. When
we have the array of pages, then we serialise the update of the object.
However, the worker should only overwrite the obj->userptr.work pointer
if and only if it is the active one. Currently we clear it for a
secondary worker with the effect that we may rarely force a second
lookup.

v2: Rebase and rename a variable to avoid 80cols
v3: Mention v2

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
---
 drivers/gpu/drm/i915/i915_gem_userptr.c | 32 ++++++++++++++++----------------
 1 file changed, 16 insertions(+), 16 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem_userptr.c b/drivers/gpu/drm/i915/i915_gem_userptr.c
index d11901d590ac..800a5394aa1e 100644
--- a/drivers/gpu/drm/i915/i915_gem_userptr.c
+++ b/drivers/gpu/drm/i915/i915_gem_userptr.c
@@ -571,25 +571,25 @@ __i915_gem_userptr_get_pages_worker(struct work_struct *_work)
 	struct get_pages_work *work = container_of(_work, typeof(*work), work);
 	struct drm_i915_gem_object *obj = work->obj;
 	struct drm_device *dev = obj->base.dev;
-	const int num_pages = obj->base.size >> PAGE_SHIFT;
+	const int npages = obj->base.size >> PAGE_SHIFT;
 	struct page **pvec;
 	int pinned, ret;
 
 	ret = -ENOMEM;
 	pinned = 0;
 
-	pvec = kmalloc(num_pages*sizeof(struct page *),
+	pvec = kmalloc(npages*sizeof(struct page *),
 		       GFP_TEMPORARY | __GFP_NOWARN | __GFP_NORETRY);
 	if (pvec == NULL)
-		pvec = drm_malloc_ab(num_pages, sizeof(struct page *));
+		pvec = drm_malloc_ab(npages, sizeof(struct page *));
 	if (pvec != NULL) {
 		struct mm_struct *mm = obj->userptr.mm->mm;
 
 		down_read(&mm->mmap_sem);
-		while (pinned < num_pages) {
+		while (pinned < npages) {
 			ret = get_user_pages(work->task, mm,
 					     obj->userptr.ptr + pinned * PAGE_SIZE,
-					     num_pages - pinned,
+					     npages - pinned,
 					     !obj->userptr.read_only, 0,
 					     pvec + pinned, NULL);
 			if (ret < 0)
@@ -601,20 +601,20 @@ __i915_gem_userptr_get_pages_worker(struct work_struct *_work)
 	}
 
 	mutex_lock(&dev->struct_mutex);
-	if (obj->userptr.work != &work->work) {
-		ret = 0;
-	} else if (pinned == num_pages) {
-		ret = __i915_gem_userptr_set_pages(obj, pvec, num_pages);
-		if (ret == 0) {
-			list_add_tail(&obj->global_list, &to_i915(dev)->mm.unbound_list);
-			obj->get_page.sg = obj->pages->sgl;
-			obj->get_page.last = 0;
-
-			pinned = 0;
+	if (obj->userptr.work == &work->work) {
+		if (pinned == npages) {
+			ret = __i915_gem_userptr_set_pages(obj, pvec, npages);
+			if (ret == 0) {
+				list_add_tail(&obj->global_list,
+					      &to_i915(dev)->mm.unbound_list);
+				obj->get_page.sg = obj->pages->sgl;
+				obj->get_page.last = 0;
+				pinned = 0;
+			}
 		}
+		obj->userptr.work = ERR_PTR(ret);
 	}
 
-	obj->userptr.work = ERR_PTR(ret);
 	obj->userptr.workers--;
 	drm_gem_object_unreference(&obj->base);
 	mutex_unlock(&dev->struct_mutex);
-- 
2.5.1

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[PATCH v4 2/3] drm/i915: Fix userptr deadlock with aliased GTT mmappings

[Chris Wilson]

Michał 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.
v5: Rearrange the error path so that is not so convoluted

Reported-by: Michał Winiarski <michal.winiarski@intel.com>
Testcase: igt/gem_userptr_blits/map-fixed*
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michał 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 | 175 ++++++++++++++++++++------------
 1 file changed, 109 insertions(+), 66 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem_userptr.c b/drivers/gpu/drm/i915/i915_gem_userptr.c
index 800a5394aa1e..505c608ade7e 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--;
@@ -627,11 +654,60 @@ __i915_gem_userptr_get_pages_worker(struct work_struct *_work)
 }
 
 static int
+__i915_gem_userptr_get_pages_schedule(struct drm_i915_gem_object *obj,
+				      bool *active)
+{
+	struct get_pages_work *work;
+
+	/* 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 (obj->userptr.workers >= I915_GEM_USERPTR_MAX_WORKERS)
+		return -EAGAIN;
+
+	work = kmalloc(sizeof(*work), GFP_KERNEL);
+	if (work == NULL)
+		return -ENOMEM;
+
+	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);
+
+	*active = true;
+	return -EAGAIN;
+}
+
+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;
+	bool active;
 
 	/* If userspace should engineer that these pages are replaced in
 	 * the vma between us binding this page into the GTT and completion
@@ -649,6 +725,18 @@ 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;
@@ -657,73 +745,27 @@ 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)
+			if (pvec == NULL) {
+				__i915_gem_userptr_set_active(obj, false);
 				return -ENOMEM;
+			}
 		}
 
 		pinned = __get_user_pages_fast(obj->userptr.ptr, num_pages,
 					       !obj->userptr.read_only, pvec);
 	}
-	if (pinned < num_pages) {
-		if (pinned < 0) {
-			ret = pinned;
-			pinned = 0;
-		} else {
-			/* 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.
-			 */
-			ret = -EAGAIN;
-			if (obj->userptr.work == NULL &&
-			    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++;
-
-					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);
-				} else
-					ret = -ENOMEM;
-			} else {
-				if (IS_ERR(obj->userptr.work)) {
-					ret = PTR_ERR(obj->userptr.work);
-					obj->userptr.work = NULL;
-				}
-			}
-		}
-	} else {
+
+	active = false;
+	if (pinned < 0)
+		ret = pinned;
+	else if (pinned < num_pages)
+		ret = __i915_gem_userptr_get_pages_schedule(obj, &active);
+	else
 		ret = __i915_gem_userptr_set_pages(obj, pvec, num_pages);
-		if (ret == 0) {
-			obj->userptr.work = NULL;
-			pinned = 0;
-		}
+	if (ret) {
+		__i915_gem_userptr_set_active(obj, active);
+		release_pages(pvec, pinned, 0);
 	}
-
-	release_pages(pvec, pinned, 0);
 	drm_free_large(pvec);
 	return ret;
 }
@@ -734,6 +776,7 @@ 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;
-- 
2.5.1

[PATCH v4 3/3] drm/i915: Use a task to cancel the userptr on invalidate_range

[Chris Wilson]

Whilst discussing possible ways to trigger an invalidate_range on a
userptr with an aliased GGTT mmapping (and so cause a struct_mutex
deadlock), the conclusion is that we can, and we must, prevent any
possible deadlock by avoiding taking the mutex at all during
invalidate_range. This has numerous advantages all of which stem from
avoid the sleeping function from inside the unknown context. In
particular, it simplifies the invalidate_range because we no longer
have to juggle the spinlock/mutex and can just hold the spinlock
for the entire walk. To compensate, we have to make get_pages a bit more
complicated in order to serialise with a pending cancel_userptr worker.
As we hold the struct_mutex, we have no choice but to return EAGAIN and
hope that the worker is then flushed before we retry after reacquiring
the struct_mutex.

The important caveat is that the invalidate_range itself is no longer
synchronous. There exists a small but definite period in time in which
the old PTE's page remain accessible via the GPU. Note however that the
physical pages themselves are not invalidated by the mmu_notifier, just
the CPU view of the address space. The impact should be limited to a
delay in pages being flushed, rather than a possibility of writing to
the wrong pages. The only race condition that this worsens is remapping
an userptr active on the GPU where fresh work may still reference the
old pages due to struct_mutex contention. Given that userspace is racing
with the GPU, it is fair to say that the results are undefined.

v2: Only queue (and importantly only take one refcnt) the worker once.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
---
 drivers/gpu/drm/i915/i915_gem_userptr.c | 148 +++++++++++++-------------------
 1 file changed, 61 insertions(+), 87 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem_userptr.c b/drivers/gpu/drm/i915/i915_gem_userptr.c
index 505c608ade7e..75bac5a5fbb4 100644
--- a/drivers/gpu/drm/i915/i915_gem_userptr.c
+++ b/drivers/gpu/drm/i915/i915_gem_userptr.c
@@ -50,7 +50,6 @@ struct i915_mmu_notifier {
 	struct mmu_notifier mn;
 	struct rb_root objects;
 	struct list_head linear;
-	unsigned long serial;
 	bool has_linear;
 };
 
@@ -59,14 +58,16 @@ struct i915_mmu_object {
 	struct interval_tree_node it;
 	struct list_head link;
 	struct drm_i915_gem_object *obj;
+	struct work_struct work;
 	bool active;
 	bool is_linear;
 };
 
-static unsigned long cancel_userptr(struct drm_i915_gem_object *obj)
+static void __cancel_userptr__worker(struct work_struct *work)
 {
+	struct i915_mmu_object *mo = container_of(work, typeof(*mo), work);
+	struct drm_i915_gem_object *obj = mo->obj;
 	struct drm_device *dev = obj->base.dev;
-	unsigned long end;
 
 	mutex_lock(&dev->struct_mutex);
 	/* Cancel any active worker and force us to re-evaluate gup */
@@ -89,46 +90,28 @@ static unsigned long cancel_userptr(struct drm_i915_gem_object *obj)
 		dev_priv->mm.interruptible = was_interruptible;
 	}
 
-	end = obj->userptr.ptr + obj->base.size;
-
 	drm_gem_object_unreference(&obj->base);
 	mutex_unlock(&dev->struct_mutex);
-
-	return end;
 }
 
-static void *invalidate_range__linear(struct i915_mmu_notifier *mn,
-				      struct mm_struct *mm,
-				      unsigned long start,
-				      unsigned long end)
+static unsigned long cancel_userptr(struct i915_mmu_object *mo)
 {
-	struct i915_mmu_object *mo;
-	unsigned long serial;
-
-restart:
-	serial = mn->serial;
-	list_for_each_entry(mo, &mn->linear, link) {
-		struct drm_i915_gem_object *obj;
-
-		if (mo->it.last < start || mo->it.start > end)
-			continue;
-
-		obj = mo->obj;
-
-		if (!mo->active ||
-		    !kref_get_unless_zero(&obj->base.refcount))
-			continue;
-
-		spin_unlock(&mn->lock);
-
-		cancel_userptr(obj);
-
-		spin_lock(&mn->lock);
-		if (serial != mn->serial)
-			goto restart;
+	unsigned long end = mo->obj->userptr.ptr + mo->obj->base.size;
+
+	/* 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 (mo->active && kref_get_unless_zero(&mo->obj->base.refcount)) {
+		schedule_work(&mo->work);
+		/* only schedule one work packet to avoid the refleak */
+		mo->active = false;
 	}
 
-	return NULL;
+	return end;
 }
 
 static void i915_gem_userptr_mn_invalidate_range_start(struct mmu_notifier *_mn,
@@ -136,45 +119,32 @@ static void i915_gem_userptr_mn_invalidate_range_start(struct mmu_notifier *_mn,
 						       unsigned long start,
 						       unsigned long end)
 {
-	struct i915_mmu_notifier *mn = container_of(_mn, struct i915_mmu_notifier, mn);
-	struct interval_tree_node *it = NULL;
-	unsigned long next = start;
-	unsigned long serial = 0;
-
-	end--; /* interval ranges are inclusive, but invalidate range is exclusive */
-	while (next < end) {
-		struct drm_i915_gem_object *obj = NULL;
-
-		spin_lock(&mn->lock);
-		if (mn->has_linear)
-			it = invalidate_range__linear(mn, mm, start, end);
-		else if (serial == mn->serial)
-			it = interval_tree_iter_next(it, next, end);
-		else
-			it = interval_tree_iter_first(&mn->objects, start, end);
-		if (it != NULL) {
-			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
-			 * 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 (mo->active &&
-			    kref_get_unless_zero(&mo->obj->base.refcount))
-				obj = mo->obj;
-
-			serial = mn->serial;
+	struct i915_mmu_notifier *mn =
+		container_of(_mn, struct i915_mmu_notifier, mn);
+	struct i915_mmu_object *mo;
+
+	/* interval ranges are inclusive, but invalidate range is exclusive */
+	end--;
+
+	spin_lock(&mn->lock);
+	if (mn->has_linear) {
+		list_for_each_entry(mo, &mn->linear, link) {
+			if (mo->it.last < start || mo->it.start > end)
+				continue;
+
+			cancel_userptr(mo);
 		}
-		spin_unlock(&mn->lock);
-		if (obj == NULL)
-			return;
+	} else {
+		struct interval_tree_node *it;
 
-		next = cancel_userptr(obj);
+		it = interval_tree_iter_first(&mn->objects, start, end);
+		while (it) {
+			mo = container_of(it, struct i915_mmu_object, it);
+			start = cancel_userptr(mo);
+			it = interval_tree_iter_next(it, start, end);
+		}
 	}
+	spin_unlock(&mn->lock);
 }
 
 static const struct mmu_notifier_ops i915_gem_userptr_notifier = {
@@ -194,7 +164,6 @@ i915_mmu_notifier_create(struct mm_struct *mm)
 	spin_lock_init(&mn->lock);
 	mn->mn.ops = &i915_gem_userptr_notifier;
 	mn->objects = RB_ROOT;
-	mn->serial = 1;
 	INIT_LIST_HEAD(&mn->linear);
 	mn->has_linear = false;
 
@@ -208,12 +177,6 @@ i915_mmu_notifier_create(struct mm_struct *mm)
 	return mn;
 }
 
-static void __i915_mmu_notifier_update_serial(struct i915_mmu_notifier *mn)
-{
-	if (++mn->serial == 0)
-		mn->serial = 1;
-}
-
 static int
 i915_mmu_notifier_add(struct drm_device *dev,
 		      struct i915_mmu_notifier *mn,
@@ -260,10 +223,9 @@ i915_mmu_notifier_add(struct drm_device *dev,
 	} else
 		interval_tree_insert(&mo->it, &mn->objects);
 
-	if (ret == 0) {
+	if (ret == 0)
 		list_add(&mo->link, &mn->linear);
-		__i915_mmu_notifier_update_serial(mn);
-	}
+
 	spin_unlock(&mn->lock);
 	mutex_unlock(&dev->struct_mutex);
 
@@ -291,7 +253,6 @@ i915_mmu_notifier_del(struct i915_mmu_notifier *mn,
 		mn->has_linear = i915_mmu_notifier_has_linear(mn);
 	else
 		interval_tree_remove(&mo->it, &mn->objects);
-	__i915_mmu_notifier_update_serial(mn);
 	spin_unlock(&mn->lock);
 }
 
@@ -358,6 +319,7 @@ i915_gem_userptr_init__mmu_notifier(struct drm_i915_gem_object *obj,
 	mo->it.start = obj->userptr.ptr;
 	mo->it.last = mo->it.start + obj->base.size - 1;
 	mo->obj = obj;
+	INIT_WORK(&mo->work, __cancel_userptr__worker);
 
 	ret = i915_mmu_notifier_add(obj->base.dev, mn, mo);
 	if (ret) {
@@ -566,10 +528,12 @@ __i915_gem_userptr_set_pages(struct drm_i915_gem_object *obj,
 	return ret;
 }
 
-static void
+static int
 __i915_gem_userptr_set_active(struct drm_i915_gem_object *obj,
 			      bool value)
 {
+	int ret = 0;
+
 	/* 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
@@ -582,12 +546,20 @@ __i915_gem_userptr_set_active(struct drm_i915_gem_object *obj,
 	 */
 #if defined(CONFIG_MMU_NOTIFIER)
 	if (obj->userptr.mmu_object == NULL)
-		return;
+		return 0;
 
 	spin_lock(&obj->userptr.mmu_object->mn->lock);
-	obj->userptr.mmu_object->active = value;
+	/* In order to serialise get_pages with an outstanding
+	 * cancel_userptr, we must drop the struct_mutex and try again.
+	 */
+	if (!value || !work_pending(&obj->userptr.mmu_object->work))
+		obj->userptr.mmu_object->active = value;
+	else
+		ret = -EAGAIN;
 	spin_unlock(&obj->userptr.mmu_object->mn->lock);
 #endif
+
+	return ret;
 }
 
 static void
@@ -736,7 +708,9 @@ i915_gem_userptr_get_pages(struct drm_i915_gem_object *obj)
 		return -EAGAIN;
 
 	/* Let the mmu-notifier know that we have begun and need cancellation */
-	__i915_gem_userptr_set_active(obj, true);
+	ret = __i915_gem_userptr_set_active(obj, true);
+	if (ret)
+		return ret;
 
 	pvec = NULL;
 	pinned = 0;
-- 
2.5.1

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Re: [Intel-gfx] [PATCH v4 2/3] drm/i915: Fix userptr deadlock with aliased GTT mmappings

[Tvrtko Ursulin]

On 09/10/2015 10:51 AM, Chris Wilson wrote:
> Michał 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.
> v5: Rearrange the error path so that is not so convoluted
>
> Reported-by: Michał Winiarski <michal.winiarski@intel.com>
> Testcase: igt/gem_userptr_blits/map-fixed*
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Michał 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 | 175 ++++++++++++++++++++------------
>   1 file changed, 109 insertions(+), 66 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_gem_userptr.c b/drivers/gpu/drm/i915/i915_gem_userptr.c
> index 800a5394aa1e..505c608ade7e 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--;
> @@ -627,11 +654,60 @@ __i915_gem_userptr_get_pages_worker(struct work_struct *_work)
>   }
>
>   static int
> +__i915_gem_userptr_get_pages_schedule(struct drm_i915_gem_object *obj,
> +				      bool *active)
> +{
> +	struct get_pages_work *work;
> +
> +	/* 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 (obj->userptr.workers >= I915_GEM_USERPTR_MAX_WORKERS)
> +		return -EAGAIN;
> +
> +	work = kmalloc(sizeof(*work), GFP_KERNEL);
> +	if (work == NULL)
> +		return -ENOMEM;
> +
> +	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);
> +
> +	*active = true;
> +	return -EAGAIN;
> +}
> +
> +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;
> +	bool active;
>
>   	/* If userspace should engineer that these pages are replaced in
>   	 * the vma between us binding this page into the GTT and completion
> @@ -649,6 +725,18 @@ 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;
> @@ -657,73 +745,27 @@ 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)
> +			if (pvec == NULL) {
> +				__i915_gem_userptr_set_active(obj, false);
>   				return -ENOMEM;
> +			}
>   		}
>
>   		pinned = __get_user_pages_fast(obj->userptr.ptr, num_pages,
>   					       !obj->userptr.read_only, pvec);
>   	}
> -	if (pinned < num_pages) {
> -		if (pinned < 0) {
> -			ret = pinned;
> -			pinned = 0;
> -		} else {
> -			/* 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.
> -			 */
> -			ret = -EAGAIN;
> -			if (obj->userptr.work == NULL &&
> -			    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++;
> -
> -					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);
> -				} else
> -					ret = -ENOMEM;
> -			} else {
> -				if (IS_ERR(obj->userptr.work)) {
> -					ret = PTR_ERR(obj->userptr.work);
> -					obj->userptr.work = NULL;
> -				}
> -			}
> -		}
> -	} else {
> +
> +	active = false;
> +	if (pinned < 0)
> +		ret = pinned;
> +	else if (pinned < num_pages)
> +		ret = __i915_gem_userptr_get_pages_schedule(obj, &active);
> +	else
>   		ret = __i915_gem_userptr_set_pages(obj, pvec, num_pages);
> -		if (ret == 0) {
> -			obj->userptr.work = NULL;
> -			pinned = 0;
> -		}
> +	if (ret) {
> +		__i915_gem_userptr_set_active(obj, active);
> +		release_pages(pvec, pinned, 0);

I think it would be safer to set pinned to zero in the pinned < 0 case, 
rather than rely on release_pages implementation knowledge. Otherwise it 
is too fragile.

If you can put that back you can put my R-B on the patch.

Regards,

Tvrtko

Re: [PATCH v4 2/3] drm/i915: Fix userptr deadlock with aliased GTT mmappings

[Tvrtko Ursulin]

On 09/10/2015 11:25 AM, Tvrtko Ursulin wrote:
>> +
>> +    active = false;
>> +    if (pinned < 0)
>> +        ret = pinned;
>> +    else if (pinned < num_pages)
>> +        ret = __i915_gem_userptr_get_pages_schedule(obj, &active);
>> +    else
>>           ret = __i915_gem_userptr_set_pages(obj, pvec, num_pages);
>> -        if (ret == 0) {
>> -            obj->userptr.work = NULL;
>> -            pinned = 0;
>> -        }
>> +    if (ret) {
>> +        __i915_gem_userptr_set_active(obj, active);
>> +        release_pages(pvec, pinned, 0);
>
> I think it would be safer to set pinned to zero in the pinned < 0 case,
> rather than rely on release_pages implementation knowledge. Otherwise it
> is too fragile.
>
> If you can put that back you can put my R-B on the patch.

Or possibly even better:

if (pinned >= 0)
	release_pages(...);

?

Tvrtko

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