dma-buf: Restore seqlock around dma_resv updates

This reverts
67c97fb79a7f ("dma-buf: add reservation_object_fences helper")
dd7a7d1ff2f1 ("drm/i915: use new reservation_object_fences helper")
0e1d8083bddb ("dma-buf: further relax reservation_object_add_shared_fence")
5d344f58da76 ("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

5 files changed, 175 insertions, 109 deletions

diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
index b3400d6524ab..433d91d710e4 100644
--- a/drivers/dma-buf/dma-buf.c
+++ b/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",
diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c
index f5142683c851..42a8f3f11681 100644
--- a/drivers/dma-buf/dma-resv.c
+++ b/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 @@ replace:
         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 @@ retry:
                         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 @@ retry:
 
                         if (!dma_fence_get_rcu(fence)) {
                                 dma_resv_list_free(dst_list);
+                                src_list = rcu_dereference(src->fence);
                                 goto retry;
                         }
 
@@ -342,18 +362,18 @@ retry:
                 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 @@ retry:
         }
 
         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 @@ retry:
 
         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 @@ retry:
                         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;
 }
diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c
index bc4ec6b20a87..76e3516484e7 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c
+++ b/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) {
diff --git a/drivers/gpu/drm/i915/gem/i915_gem_busy.c b/drivers/gpu/drm/i915/gem/i915_gem_busy.c
index a2aff1d8290e..3d4f5775a4ba 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_busy.c
+++ b/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();
diff --git a/include/linux/dma-resv.h b/include/linux/dma-resv.h
index 38f2802afabb..ee50d10f052b 100644
--- a/include/linux/dma-resv.h
+++ b/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;
@@ -78,24 +81,6 @@ struct dma_resv {
 #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
  * @obj: the reservation object
@@ -110,53 +95,6 @@ static inline struct dma_resv_list *dma_resv_get_list(struct dma_resv *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
  * @ctx: the locking context
@@ -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);