On 01/08/2019 17:16, Chris Wilson wrote:
Quoting Lionel Landwerlin (2019-08-01 15:29:34)
On 31/07/2019 23:03, Chris Wilson wrote:
Quoting Lionel Landwerlin (2019-07-31 15:07:33)
-static struct drm_syncobj **
-get_fence_array(struct drm_i915_gem_execbuffer2 *args,
- struct drm_file *file)
+static struct i915_eb_fences *
+get_timeline_fence_array(struct i915_execbuffer *eb, int *out_n_fences)
+{
+ struct drm_i915_gem_execbuffer_ext_timeline_fences *timeline_fences =
+ &eb->extensions.timeline_fences;
+ struct drm_i915_gem_exec_fence __user *user_fences;
+ struct i915_eb_fences *fences;
+ u64 __user *user_values;
+ u64 num_fences, num_user_fences = timeline_fences->fence_count;
+ unsigned long n;
+ int err;
+
+ /* Check multiplication overflow for access_ok() and kvmalloc_array() */
+ BUILD_BUG_ON(sizeof(size_t) > sizeof(unsigned long));
+ if (num_user_fences > min_t(unsigned long,
+ ULONG_MAX / sizeof(*user_fences),
+ SIZE_MAX / sizeof(*fences)))
+ return ERR_PTR(-EINVAL);
+
+ user_fences = u64_to_user_ptr(timeline_fences->handles_ptr);
+ if (!access_ok(user_fences, num_user_fences * sizeof(*user_fences)))
+ return ERR_PTR(-EFAULT);
+
+ user_values = u64_to_user_ptr(timeline_fences->values_ptr);
+ if (!access_ok(user_values, num_user_fences * sizeof(*user_values)))
+ return ERR_PTR(-EFAULT);
+
+ fences = kvmalloc_array(num_user_fences, sizeof(*fences),
+ __GFP_NOWARN | GFP_KERNEL);
+ if (!fences)
+ return ERR_PTR(-ENOMEM);
+
+ BUILD_BUG_ON(~(ARCH_KMALLOC_MINALIGN - 1) &
+ ~__I915_EXEC_FENCE_UNKNOWN_FLAGS);
+
+ for (n = 0, num_fences = 0; n < timeline_fences->fence_count; n++) {
+ struct drm_i915_gem_exec_fence user_fence;
+ struct drm_syncobj *syncobj;
+ struct dma_fence *fence = NULL;
+ u64 point;
+
+ if (__copy_from_user(&user_fence, user_fences++, sizeof(user_fence))) {
+ err = -EFAULT;
+ goto err;
+ }
+
+ if (user_fence.flags & __I915_EXEC_FENCE_UNKNOWN_FLAGS) {
+ err = -EINVAL;
+ goto err;
+ }
+
+ if (__get_user(point, user_values++)) {
+ err = -EFAULT;
+ goto err;
+ }
+
+ syncobj = drm_syncobj_find(eb->file, user_fence.handle);
+ if (!syncobj) {
+ DRM_DEBUG("Invalid syncobj handle provided\n");
+ err = -ENOENT;
+ goto err;
+ }
+
+ if (user_fence.flags & I915_EXEC_FENCE_WAIT) {
+ fence = drm_syncobj_fence_get(syncobj);
+ if (!fence) {
+ DRM_DEBUG("Syncobj handle has no fence\n");
+ drm_syncobj_put(syncobj);
+ err = -EINVAL;
+ goto err;
+ }
+
+ err = dma_fence_chain_find_seqno(&fence, point);
+ if (err) {
+ DRM_DEBUG("Syncobj handle missing requested point %llu\n", point);
+ drm_syncobj_put(syncobj);
+ goto err;
+ }
+
+ /* A point might have been signaled already and
+ * garbage collected from the timeline. In this case
+ * just ignore the point and carry on.
+ */
+ if (!fence) {
+ drm_syncobj_put(syncobj);
+ continue;
+ }
+ }
+
+ /*
+ * For timeline syncobjs we need to preallocate chains for
+ * later signaling.
+ */
+ if (point != 0 && user_fence.flags & I915_EXEC_FENCE_SIGNAL) {
if (dma_fence_chain_find_seqno() == 0)
return -EINVAL;
as an early sanity check?
+ fences[num_fences].chain_fence =
+ kmalloc(sizeof(*fences[num_fences].chain_fence),
+ GFP_KERNEL);
+ if (!fences[num_fences].chain_fence) {
+ dma_fence_put(fence);
+ drm_syncobj_put(syncobj);
+ err = -ENOMEM;
+ DRM_DEBUG("Unable to alloc chain_fence\n");
+ goto err;
+ }
+ } else {
+ fences[num_fences].chain_fence = NULL;
+ }
+
+ fences[num_fences].syncobj = ptr_pack_bits(syncobj, user_fence.flags, 2);
+ fences[num_fences].dma_fence = fence;
+ fences[num_fences].value = point;
+ num_fences++;
+ }
+
+ *out_n_fences = num_fences;
+
+ return fences;
+
+err:
+ __free_fence_array(fences, num_fences);
+ return ERR_PTR(err);
+}
@@ -2329,9 +2484,9 @@ await_fence_array(struct i915_execbuffer *eb,
static void
signal_fence_array(struct i915_execbuffer *eb,
- struct drm_syncobj **fences)
+ struct i915_eb_fences *fences,
+ int nfences)
{
- const unsigned int nfences = eb->args->num_cliprects;
struct dma_fence * const fence = &eb->request->fence;
unsigned int n;
@@ -2339,15 +2494,46 @@ signal_fence_array(struct i915_execbuffer *eb,
struct drm_syncobj *syncobj;
unsigned int flags;
- syncobj = ptr_unpack_bits(fences[n], &flags, 2);
+ syncobj = ptr_unpack_bits(fences[n].syncobj, &flags, 2);
if (!(flags & I915_EXEC_FENCE_SIGNAL))
continue;
- drm_syncobj_replace_fence(syncobj, fence);
+ if (fences[n].chain_fence) {
+ drm_syncobj_add_point(syncobj, fences[n].chain_fence,
+ fence, fences[n].value);
+ /*
+ * The chain's ownership is transfered to the
+ * timeline.
+ */
+ fences[n].chain_fence = NULL;
+ } else {
+ drm_syncobj_replace_fence(syncobj, fence);
+ }
}
}
I think I have convinced myself that with the split between wait before,
signal after combined with the rule that seqno point along the syncobj
are monotonic, you should not be able to generate an AB-BA deadlock
between concurrent clients.
Can you come up with an example that would deadlock?
Timeline holds 2,1; a wait on 2 will fail with -EINVAL to userspace.
(Though possibly perfectly valid behaviour on the part of the user.)
Timeline holds 2, with 1 being submitted. A wait on 1 waits on 2
instead. If 1 gains a dependency on A (e.g. bad userspace or an
implicit fence, it's a concurrent wait/submit so expect the worst, i.e.
userspace has to be racing with itself to get into this mess), you
now have a deadlock. (The assumption being that the syncpt along the
timeline are themselves not strictly ordered, and considering they are
external syncobj, that seems like a reasonable generalisation.)
-Chris
Hey Chris,
Does this discussion around how the dma-fence-chain works prevent those
AB-BA deadlocks? :
https://lists.freedesktop.org/archives/dri-devel/2019-August/229289.html
Essentially any point added to the timeline will not be signaled until
all previous points are.
This is ensured by the dma-fence-chain node which waits for all previous
dma-fence-chains nodes to be signal and its own wrapped fence (in our
case an i915 one).
Thanks,
-Lionel
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