dma-buf: Restore seqlock around dma_resv updates

This reverts
67c97fb7 ("dma-buf: add reservation_object_fences helper")
dd7a7d1f ("drm/i915: use new reservation_object_fences helper")
0e1d8083 ("dma-buf: further relax reservation_object_add_shared_fence")
5d344f58 ("dma-buf: nuke reservation_object seq number")

The scenario that defeats simply grabbing a set of shared/exclusive
fences and using them blissfully under RCU is that any of those fences
may be reallocated by a SLAB_TYPESAFE_BY_RCU fence slab cache. In this
scenario, while keeping the rcu_read_lock we need to establish that no
fence was changed in the dma_resv after a read (or full) memory barrier.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190814182401.25009-1-chris@chris-wilson.co.uk

drivers/dma-buf/dma-buf.c

@@ -199,7 +199,7 @@ static __poll_t dma_buf_poll(struct file *file, poll_table *poll)
 	struct dma_resv_list *fobj;
 	struct dma_fence *fence_excl;
 	__poll_t events;
-	unsigned shared_count;
+	unsigned shared_count, seq;
 
 	dmabuf = file->private_data;
 	if (!dmabuf || !dmabuf->resv)
@@ -213,8 +213,21 @@ static __poll_t dma_buf_poll(struct file *file, poll_table *poll)
 	if (!events)
 		return 0;
 
+retry:
+	seq = read_seqcount_begin(&resv->seq);
 	rcu_read_lock();
-	dma_resv_fences(resv, &fence_excl, &fobj, &shared_count);
+
+	fobj = rcu_dereference(resv->fence);
+	if (fobj)
+		shared_count = fobj->shared_count;
+	else
+		shared_count = 0;
+	fence_excl = rcu_dereference(resv->fence_excl);
+	if (read_seqcount_retry(&resv->seq, seq)) {
+		rcu_read_unlock();
+		goto retry;
+	}
+
 	if (fence_excl && (!(events & EPOLLOUT) || shared_count == 0)) {
 		struct dma_buf_poll_cb_t *dcb = &dmabuf->cb_excl;
 		__poll_t pevents = EPOLLIN;
@@ -1144,6 +1157,7 @@ static int dma_buf_debug_show(struct seq_file *s, void *unused)
 	struct dma_resv *robj;
 	struct dma_resv_list *fobj;
 	struct dma_fence *fence;
+	unsigned seq;
 	int count = 0, attach_count, shared_count, i;
 	size_t size = 0;
 
@@ -1174,9 +1188,16 @@ static int dma_buf_debug_show(struct seq_file *s, void *unused)
 				buf_obj->name ?: "");
 
 		robj = buf_obj->resv;
-		rcu_read_lock();
-		dma_resv_fences(robj, &fence, &fobj, &shared_count);
-		rcu_read_unlock();
+		while (true) {
+			seq = read_seqcount_begin(&robj->seq);
+			rcu_read_lock();
+			fobj = rcu_dereference(robj->fence);
+			shared_count = fobj ? fobj->shared_count : 0;
+			fence = rcu_dereference(robj->fence_excl);
+			if (!read_seqcount_retry(&robj->seq, seq))
+				break;
+			rcu_read_unlock();
+		}
 
 		if (fence)
 			seq_printf(s, "\tExclusive fence: %s %s %ssignalled\n",

drivers/dma-buf/dma-resv.c

@@ -49,6 +49,12 @@
 DEFINE_WD_CLASS(reservation_ww_class);
 EXPORT_SYMBOL(reservation_ww_class);
 
+struct lock_class_key reservation_seqcount_class;
+EXPORT_SYMBOL(reservation_seqcount_class);
+
+const char reservation_seqcount_string[] = "reservation_seqcount";
+EXPORT_SYMBOL(reservation_seqcount_string);
+
 /**
  * dma_resv_list_alloc - allocate fence list
  * @shared_max: number of fences we need space for
@@ -96,6 +102,9 @@ static void dma_resv_list_free(struct dma_resv_list *list)
 void dma_resv_init(struct dma_resv *obj)
 {
 	ww_mutex_init(&obj->lock, &reservation_ww_class);
+
+	__seqcount_init(&obj->seq, reservation_seqcount_string,
+			&reservation_seqcount_class);
 	RCU_INIT_POINTER(obj->fence, NULL);
 	RCU_INIT_POINTER(obj->fence_excl, NULL);
 }
@@ -225,6 +234,9 @@ void dma_resv_add_shared_fence(struct dma_resv *obj, struct dma_fence *fence)
 	fobj = dma_resv_get_list(obj);
 	count = fobj->shared_count;
 
+	preempt_disable();
+	write_seqcount_begin(&obj->seq);
+
 	for (i = 0; i < count; ++i) {
 
 		old = rcu_dereference_protected(fobj->shared[i],
@@ -242,6 +254,9 @@ void dma_resv_add_shared_fence(struct dma_resv *obj, struct dma_fence *fence)
 	RCU_INIT_POINTER(fobj->shared[i], fence);
 	/* pointer update must be visible before we extend the shared_count */
 	smp_store_mb(fobj->shared_count, count);
+
+	write_seqcount_end(&obj->seq);
+	preempt_enable();
 	dma_fence_put(old);
 }
 EXPORT_SYMBOL(dma_resv_add_shared_fence);
@@ -269,10 +284,12 @@ void dma_resv_add_excl_fence(struct dma_resv *obj, struct dma_fence *fence)
 		dma_fence_get(fence);
 
 	preempt_disable();
-	rcu_assign_pointer(obj->fence_excl, fence);
-	/* pointer update must be visible before we modify the shared_count */
+	write_seqcount_begin(&obj->seq);
+	/* write_seqcount_begin provides the necessary memory barrier */
+	RCU_INIT_POINTER(obj->fence_excl, fence);
 	if (old)
-		smp_store_mb(old->shared_count, 0);
+		old->shared_count = 0;
+	write_seqcount_end(&obj->seq);
 	preempt_enable();
 
 	/* inplace update, no shared fences */
@@ -295,15 +312,17 @@ int dma_resv_copy_fences(struct dma_resv *dst, struct dma_resv *src)
 {
 	struct dma_resv_list *src_list, *dst_list;
 	struct dma_fence *old, *new;
-	unsigned int i, shared_count;
+	unsigned i;
 
 	dma_resv_assert_held(dst);
 
 	rcu_read_lock();
+	src_list = rcu_dereference(src->fence);
 
 retry:
-	dma_resv_fences(src, &new, &src_list, &shared_count);
-	if (shared_count) {
+	if (src_list) {
+		unsigned shared_count = src_list->shared_count;
+
 		rcu_read_unlock();
 
 		dst_list = dma_resv_list_alloc(shared_count);
@@ -311,14 +330,14 @@ int dma_resv_copy_fences(struct dma_resv *dst, struct dma_resv *src)
 			return -ENOMEM;
 
 		rcu_read_lock();
-		dma_resv_fences(src, &new, &src_list, &shared_count);
-		if (!src_list || shared_count > dst_list->shared_max) {
+		src_list = rcu_dereference(src->fence);
+		if (!src_list || src_list->shared_count > shared_count) {
 			kfree(dst_list);
 			goto retry;
 		}
 
 		dst_list->shared_count = 0;
-		for (i = 0; i < shared_count; ++i) {
+		for (i = 0; i < src_list->shared_count; ++i) {
 			struct dma_fence *fence;
 
 			fence = rcu_dereference(src_list->shared[i]);
@@ -328,6 +347,7 @@ int dma_resv_copy_fences(struct dma_resv *dst, struct dma_resv *src)
 
 			if (!dma_fence_get_rcu(fence)) {
 				dma_resv_list_free(dst_list);
+				src_list = rcu_dereference(src->fence);
 				goto retry;
 			}
 
@@ -342,18 +362,18 @@ int dma_resv_copy_fences(struct dma_resv *dst, struct dma_resv *src)
 		dst_list = NULL;
 	}
 
-	if (new && !dma_fence_get_rcu(new)) {
-		dma_resv_list_free(dst_list);
-		goto retry;
-	}
+	new = dma_fence_get_rcu_safe(&src->fence_excl);
 	rcu_read_unlock();
 
 	src_list = dma_resv_get_list(dst);
 	old = dma_resv_get_excl(dst);
 
 	preempt_disable();
-	rcu_assign_pointer(dst->fence_excl, new);
-	rcu_assign_pointer(dst->fence, dst_list);
+	write_seqcount_begin(&dst->seq);
+	/* write_seqcount_begin provides the necessary memory barrier */
+	RCU_INIT_POINTER(dst->fence_excl, new);
+	RCU_INIT_POINTER(dst->fence, dst_list);
+	write_seqcount_end(&dst->seq);
 	preempt_enable();
 
 	dma_resv_list_free(src_list);
@@ -388,18 +408,19 @@ int dma_resv_get_fences_rcu(struct dma_resv *obj,
 
 	do {
 		struct dma_resv_list *fobj;
-		unsigned int i;
+		unsigned int i, seq;
 		size_t sz = 0;
 
-		i = 0;
+		shared_count = i = 0;
 
 		rcu_read_lock();
-		dma_resv_fences(obj, &fence_excl, &fobj,
-					  &shared_count);
+		seq = read_seqcount_begin(&obj->seq);
 
+		fence_excl = rcu_dereference(obj->fence_excl);
 		if (fence_excl && !dma_fence_get_rcu(fence_excl))
 			goto unlock;
 
+		fobj = rcu_dereference(obj->fence);
 		if (fobj)
 			sz += sizeof(*shared) * fobj->shared_max;
 
@@ -427,6 +448,7 @@ int dma_resv_get_fences_rcu(struct dma_resv *obj,
 				break;
 			}
 			shared = nshared;
+			shared_count = fobj ? fobj->shared_count : 0;
 			for (i = 0; i < shared_count; ++i) {
 				shared[i] = rcu_dereference(fobj->shared[i]);
 				if (!dma_fence_get_rcu(shared[i]))
@@ -434,7 +456,7 @@ int dma_resv_get_fences_rcu(struct dma_resv *obj,
 			}
 		}
 
-		if (i != shared_count) {
+		if (i != shared_count || read_seqcount_retry(&obj->seq, seq)) {
 			while (i--)
 				dma_fence_put(shared[i]);
 			dma_fence_put(fence_excl);
@@ -478,17 +500,18 @@ long dma_resv_wait_timeout_rcu(struct dma_resv *obj,
 			       bool wait_all, bool intr,
 			       unsigned long timeout)
 {
-	struct dma_resv_list *fobj;
 	struct dma_fence *fence;
-	unsigned shared_count;
+	unsigned seq, shared_count;
 	long ret = timeout ? timeout : 1;
 	int i;
 
 retry:
+	shared_count = 0;
+	seq = read_seqcount_begin(&obj->seq);
 	rcu_read_lock();
 	i = -1;
 
-	dma_resv_fences(obj, &fence, &fobj, &shared_count);
+	fence = rcu_dereference(obj->fence_excl);
 	if (fence && !test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags)) {
 		if (!dma_fence_get_rcu(fence))
 			goto unlock_retry;
@@ -503,6 +526,11 @@ long dma_resv_wait_timeout_rcu(struct dma_resv *obj,
 	}
 
 	if (wait_all) {
+		struct dma_resv_list *fobj = rcu_dereference(obj->fence);
+
+		if (fobj)
+			shared_count = fobj->shared_count;
+
 		for (i = 0; !fence && i < shared_count; ++i) {
 			struct dma_fence *lfence = rcu_dereference(fobj->shared[i]);
 
@@ -525,6 +553,11 @@ long dma_resv_wait_timeout_rcu(struct dma_resv *obj,
 
 	rcu_read_unlock();
 	if (fence) {
+		if (read_seqcount_retry(&obj->seq, seq)) {
+			dma_fence_put(fence);
+			goto retry;
+		}
+
 		ret = dma_fence_wait_timeout(fence, intr, ret);
 		dma_fence_put(fence);
 		if (ret > 0 && wait_all && (i + 1 < shared_count))
@@ -567,19 +600,23 @@ static inline int dma_resv_test_signaled_single(struct dma_fence *passed_fence)
  */
 bool dma_resv_test_signaled_rcu(struct dma_resv *obj, bool test_all)
 {
-	struct dma_resv_list *fobj;
-	struct dma_fence *fence_excl;
-	unsigned shared_count;
+	unsigned seq, shared_count;
 	int ret;
 
 	rcu_read_lock();
 retry:
 	ret = true;
+	shared_count = 0;
+	seq = read_seqcount_begin(&obj->seq);
 
-	dma_resv_fences(obj, &fence_excl, &fobj, &shared_count);
 	if (test_all) {
 		unsigned i;
 
+		struct dma_resv_list *fobj = rcu_dereference(obj->fence);
+
+		if (fobj)
+			shared_count = fobj->shared_count;
+
 		for (i = 0; i < shared_count; ++i) {
 			struct dma_fence *fence = rcu_dereference(fobj->shared[i]);
 
@@ -589,14 +626,24 @@ bool dma_resv_test_signaled_rcu(struct dma_resv *obj, bool test_all)
 			else if (!ret)
 				break;
 		}
-	}
 
-	if (!shared_count && fence_excl) {
-		ret = dma_resv_test_signaled_single(fence_excl);
-		if (ret < 0)
+		if (read_seqcount_retry(&obj->seq, seq))
 			goto retry;
 	}
 
+	if (!shared_count) {
+		struct dma_fence *fence_excl = rcu_dereference(obj->fence_excl);
+
+		if (fence_excl) {
+			ret = dma_resv_test_signaled_single(fence_excl);
+			if (ret < 0)
+				goto retry;
+
+			if (read_seqcount_retry(&obj->seq, seq))
+				goto retry;
+		}
+	}
+
 	rcu_read_unlock();
 	return ret;
 }

drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c

@@ -251,7 +251,12 @@ static int amdgpu_amdkfd_remove_eviction_fence(struct amdgpu_bo *bo,
 	new->shared_max = old->shared_max;
 	new->shared_count = k;
 
-	rcu_assign_pointer(resv->fence, new);
+	/* Install the new fence list, seqcount provides the barriers */
+	preempt_disable();
+	write_seqcount_begin(&resv->seq);
+	RCU_INIT_POINTER(resv->fence, new);
+	write_seqcount_end(&resv->seq);
+	preempt_enable();
 
 	/* Drop the references to the removed fences or move them to ef_list */
 	for (i = j, k = 0; i < old->shared_count; ++i) {

drivers/gpu/drm/i915/gem/i915_gem_busy.c

@@ -83,8 +83,7 @@ i915_gem_busy_ioctl(struct drm_device *dev, void *data,
 	struct drm_i915_gem_busy *args = data;
 	struct drm_i915_gem_object *obj;
 	struct dma_resv_list *list;
-	unsigned int i, shared_count;
-	struct dma_fence *excl;
+	unsigned int seq;
 	int err;
 
 	err = -ENOENT;
@@ -110,18 +109,29 @@ i915_gem_busy_ioctl(struct drm_device *dev, void *data,
 	 * to report the overall busyness. This is what the wait-ioctl does.
 	 *
 	 */
-	dma_resv_fences(obj->base.resv, &excl, &list, &shared_count);
+retry:
+	seq = raw_read_seqcount(&obj->base.resv->seq);
 
 	/* Translate the exclusive fence to the READ *and* WRITE engine */
-	args->busy = busy_check_writer(excl);
+	args->busy =
+		busy_check_writer(rcu_dereference(obj->base.resv->fence_excl));
 
 	/* Translate shared fences to READ set of engines */
-	for (i = 0; i < shared_count; ++i) {
-		struct dma_fence *fence = rcu_dereference(list->shared[i]);
+	list = rcu_dereference(obj->base.resv->fence);
+	if (list) {
+		unsigned int shared_count = list->shared_count, i;
 
-		args->busy |= busy_check_reader(fence);
+		for (i = 0; i < shared_count; ++i) {
+			struct dma_fence *fence =
+				rcu_dereference(list->shared[i]);
+
+			args->busy |= busy_check_reader(fence);
+		}
 	}
 
+	if (args->busy && read_seqcount_retry(&obj->base.resv->seq, seq))
+		goto retry;
+
 	err = 0;
 out:
 	rcu_read_unlock();

include/linux/dma-resv.h

@@ -46,6 +46,8 @@
 #include <linux/rcupdate.h>
 
 extern struct ww_class reservation_ww_class;
+extern struct lock_class_key reservation_seqcount_class;
+extern const char reservation_seqcount_string[];
 
 /**
  * struct dma_resv_list - a list of shared fences
@@ -69,6 +71,7 @@ struct dma_resv_list {
  */
 struct dma_resv {
 	struct ww_mutex lock;
+	seqcount_t seq;
 
 	struct dma_fence __rcu *fence_excl;
 	struct dma_resv_list __rcu *fence;
@@ -77,24 +80,6 @@ struct dma_resv {
 #define dma_resv_held(obj) lockdep_is_held(&(obj)->lock.base)
 #define dma_resv_assert_held(obj) lockdep_assert_held(&(obj)->lock.base)
 
-/**
- * dma_resv_get_excl - get the reservation object's
- * exclusive fence, with update-side lock held
- * @obj: the reservation object
- *
- * Returns the exclusive fence (if any).  Does NOT take a
- * reference. Writers must hold obj->lock, readers may only
- * hold a RCU read side lock.
- *
- * RETURNS
- * The exclusive fence or NULL
- */
-static inline struct dma_fence *dma_resv_get_excl(struct dma_resv *obj)
-{
-	return rcu_dereference_protected(obj->fence_excl,
-					 dma_resv_held(obj));
-}
-
 /**
  * dma_resv_get_list - get the reservation object's
  * shared fence list, with update-side lock held
@@ -109,53 +94,6 @@ static inline struct dma_resv_list *dma_resv_get_list(struct dma_resv *obj)
 					 dma_resv_held(obj));
 }
 
-/**
- * dma_resv_fences - read consistent fence pointers
- * @obj: reservation object where we get the fences from
- * @excl: pointer for the exclusive fence
- * @list: pointer for the shared fence list
- *
- * Make sure we have a consisten exclusive fence and shared fence list.
- * Must be called with rcu read side lock held.
- */
-static inline void dma_resv_fences(struct dma_resv *obj,
-				   struct dma_fence **excl,
-				   struct dma_resv_list **list,
-				   u32 *shared_count)
-{
-	do {
-		*excl = rcu_dereference(obj->fence_excl);
-		*list = rcu_dereference(obj->fence);
-		*shared_count = *list ? (*list)->shared_count : 0;
-		smp_rmb(); /* See dma_resv_add_excl_fence */
-	} while (rcu_access_pointer(obj->fence_excl) != *excl);
-}
-
-/**
- * dma_resv_get_excl_rcu - get the reservation object's
- * exclusive fence, without lock held.
- * @obj: the reservation object
- *
- * If there is an exclusive fence, this atomically increments it's
- * reference count and returns it.
- *
- * RETURNS
- * The exclusive fence or NULL if none
- */
-static inline struct dma_fence *dma_resv_get_excl_rcu(struct dma_resv *obj)
-{
-	struct dma_fence *fence;
-
-	if (!rcu_access_pointer(obj->fence_excl))
-		return NULL;
-
-	rcu_read_lock();
-	fence = dma_fence_get_rcu_safe(&obj->fence_excl);
-	rcu_read_unlock();
-
-	return fence;
-}
-
 /**
  * dma_resv_lock - lock the reservation object
  * @obj: the reservation object
@@ -290,6 +228,51 @@ static inline void dma_resv_unlock(struct dma_resv *obj)
 	ww_mutex_unlock(&obj->lock);
 }
 
+/**
+ * dma_resv_get_excl - get the reservation object's
+ * exclusive fence, with update-side lock held
+ * @obj: the reservation object
+ *
+ * Returns the exclusive fence (if any).  Does NOT take a
+ * reference. Writers must hold obj->lock, readers may only
+ * hold a RCU read side lock.
+ *
+ * RETURNS
+ * The exclusive fence or NULL
+ */
+static inline struct dma_fence *
+dma_resv_get_excl(struct dma_resv *obj)
+{
+	return rcu_dereference_protected(obj->fence_excl,
+					 dma_resv_held(obj));
+}
+
+/**
+ * dma_resv_get_excl_rcu - get the reservation object's
+ * exclusive fence, without lock held.
+ * @obj: the reservation object
+ *
+ * If there is an exclusive fence, this atomically increments it's
+ * reference count and returns it.
+ *
+ * RETURNS
+ * The exclusive fence or NULL if none
+ */
+static inline struct dma_fence *
+dma_resv_get_excl_rcu(struct dma_resv *obj)
+{
+	struct dma_fence *fence;
+
+	if (!rcu_access_pointer(obj->fence_excl))
+		return NULL;
+
+	rcu_read_lock();
+	fence = dma_fence_get_rcu_safe(&obj->fence_excl);
+	rcu_read_unlock();
+
+	return fence;
+}
+
 void dma_resv_init(struct dma_resv *obj);
 void dma_resv_fini(struct dma_resv *obj);
 int dma_resv_reserve_shared(struct dma_resv *obj, unsigned int num_fences);