Am 17.08.19 um 16:47 schrieb Chris Wilson:
> 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.
>
> Suggested-by: Christian König <christian.koe...@amd.com>
> Signed-off-by: Chris Wilson <ch...@chris-wilson.co.uk>
> Cc: Christian König <christian.koe...@amd.com>
> ---
> 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 | 17 +++++++++++++----
> 4 files changed, 32 insertions(+), 17 deletions(-)
>
> diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
> index 89d96e3e6df6..2c21115b1a37 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);
Stashing the timestamp instead is probably less bytes to modify.
> +
> 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;
> @@ -234,7 +235,8 @@ 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..6c36502a0562 100644
> --- a/include/linux/dma-fence.h
> +++ b/include/linux/dma-fence.h
> @@ -65,17 +65,26 @@ 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.
> */
> union {
> struct rcu_head rcu;
> struct list_head cb_list;
> + ktime_t timestamp;
Maybe sort this cb_list, timestamp, rcu to better note in which order it
is used in the lifetime of a fence.
Apart from that looks good to me,
Christian.
> };
> u64 context;
> u64 seqno;
> - ktime_t timestamp;
> unsigned long flags;
> struct kref refcount;
> int error;
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