The timestamp and the cb_list are mutually exclusive, the cb_list can
only be added to prior to being signaled (and once signaled we drain),
while the timestamp is only valid upon being signaled. Both the
timestamp and the cb_list are only valid while the fence is alive, and
as soon as no references are held can be replaced by the rcu_head.
By reusing the union for the timestamp, we squeeze the base dma_fence
struct to 64 bytes on x86-64.
v2: Sort the union chronologically
Suggested-by: Christian König <christian.koenig@xxxxxxx>
Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Cc: Christian König <christian.koenig@xxxxxxx>
---
drivers/dma-buf/dma-fence.c | 16 +++++++-------
drivers/gpu/drm/i915/gt/intel_breadcrumbs.c | 13 ++++++------
drivers/gpu/drm/vmwgfx/vmwgfx_fence.c | 3 +++
include/linux/dma-fence.h | 23 ++++++++++++++++-----
4 files changed, 37 insertions(+), 18 deletions(-)
diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
index 8a6d0250285d..2c136aee3e79 100644
--- a/drivers/dma-buf/dma-fence.c
+++ b/drivers/dma-buf/dma-fence.c
@@ -129,6 +129,7 @@ EXPORT_SYMBOL(dma_fence_context_alloc);
int dma_fence_signal_locked(struct dma_fence *fence)
{
struct dma_fence_cb *cur, *tmp;
+ struct list_head cb_list;
lockdep_assert_held(fence->lock);
@@ -136,16 +137,16 @@ int dma_fence_signal_locked(struct dma_fence *fence)
&fence->flags)))
return -EINVAL;
+ /* Stash the cb_list before replacing it with the timestamp */
+ list_replace(&fence->cb_list, &cb_list);
+
fence->timestamp = ktime_get();
set_bit(DMA_FENCE_FLAG_TIMESTAMP_BIT, &fence->flags);
trace_dma_fence_signaled(fence);
- if (!list_empty(&fence->cb_list)) {
- list_for_each_entry_safe(cur, tmp, &fence->cb_list, node) {
- INIT_LIST_HEAD(&cur->node);
- cur->func(fence, cur);
- }
- INIT_LIST_HEAD(&fence->cb_list);
+ list_for_each_entry_safe(cur, tmp, &cb_list, node) {
+ INIT_LIST_HEAD(&cur->node);
+ cur->func(fence, cur);
}
return 0;
@@ -231,7 +232,8 @@ void dma_fence_release(struct kref *kref)
trace_dma_fence_destroy(fence);
- if (WARN(!list_empty(&fence->cb_list),
+ if (WARN(!list_empty(&fence->cb_list) &&
+ !test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags),
"Fence %s:%s:%llx:%llx released with pending signals!\n",
fence->ops->get_driver_name(fence),
fence->ops->get_timeline_name(fence),
diff --git a/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c b/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
index 2bc9c460e78d..09c68dda2098 100644
--- a/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
+++ b/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
@@ -114,18 +114,18 @@ __dma_fence_signal__timestamp(struct dma_fence *fence, ktime_t timestamp)
}
static void
-__dma_fence_signal__notify(struct dma_fence *fence)
+__dma_fence_signal__notify(struct dma_fence *fence,
+ const struct list_head *list)
{
struct dma_fence_cb *cur, *tmp;
lockdep_assert_held(fence->lock);
lockdep_assert_irqs_disabled();
- list_for_each_entry_safe(cur, tmp, &fence->cb_list, node) {
+ list_for_each_entry_safe(cur, tmp, list, node) {
INIT_LIST_HEAD(&cur->node);
cur->func(fence, cur);
}
- INIT_LIST_HEAD(&fence->cb_list);
}
void intel_engine_breadcrumbs_irq(struct intel_engine_cs *engine)
@@ -187,11 +187,12 @@ void intel_engine_breadcrumbs_irq(struct intel_engine_cs *engine)
list_for_each_safe(pos, next, &signal) {
struct i915_request *rq =
list_entry(pos, typeof(*rq), signal_link);
-
- __dma_fence_signal__timestamp(&rq->fence, timestamp);
+ struct list_head cb_list;
spin_lock(&rq->lock);
- __dma_fence_signal__notify(&rq->fence);
+ list_replace(&rq->fence.cb_list, &cb_list);
+ __dma_fence_signal__timestamp(&rq->fence, timestamp);
+ __dma_fence_signal__notify(&rq->fence, &cb_list);
spin_unlock(&rq->lock);
i915_request_put(rq);
diff --git a/drivers/gpu/drm/vmwgfx/vmwgfx_fence.c b/drivers/gpu/drm/vmwgfx/vmwgfx_fence.c
index 434dfadb0e52..178a6cd1a06f 100644
--- a/drivers/gpu/drm/vmwgfx/vmwgfx_fence.c
+++ b/drivers/gpu/drm/vmwgfx/vmwgfx_fence.c
@@ -185,6 +185,9 @@ static long vmw_fence_wait(struct dma_fence *f, bool intr, signed long timeout)
spin_lock(f->lock);
+ if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &f->flags))
+ goto out;
+
if (intr && signal_pending(current)) {
ret = -ERESTARTSYS;
goto out;
diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h
index 2ce4d877d33e..8b4a5aaa6848 100644
--- a/include/linux/dma-fence.h
+++ b/include/linux/dma-fence.h
@@ -65,17 +65,30 @@ struct dma_fence_cb;
struct dma_fence {
spinlock_t *lock;
const struct dma_fence_ops *ops;
- /* We clear the callback list on kref_put so that by the time we
- * release the fence it is unused. No one should be adding to the cb_list
- * that they don't themselves hold a reference for.
+ /*
+ * We clear the callback list on kref_put so that by the time we
+ * release the fence it is unused. No one should be adding to the
+ * cb_list that they don't themselves hold a reference for.
+ *
+ * The lifetime of the timestamp is similarly tied to both the
+ * rcu freelist and the cb_list. The timestamp is only set upon
+ * signaling while simultaneously notifying the cb_list. Ergo, we
+ * only use either the cb_list of timestamp. Upon destruction,
+ * neither are accessible, and so we can use the rcu. This means
+ * that the cb_list is *only* valid until the signal bit is set,
+ * and to read either you *must* hold a reference to the fence.
+ *
+ * Listed in chronological order.
*/
union {
- struct rcu_head rcu;
struct list_head cb_list;
+ /* @cb_list replaced by @timestamp on dma_fence_signal() */
+ ktime_t timestamp;
+ /* @timestamp replaced by @rcu on dma_fence_release() */
+ struct rcu_head rcu;
};
u64 context;
u64 seqno;
- ktime_t timestamp;
unsigned long flags;
struct kref refcount;
int error;
--
2.23.0.rc1
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