Re: [Intel-gfx] [PATCH 03/26] dma-buf: use new iterator in dma_resv_copy_fences

Daniel Vetter
2021-09-17T14:35:27Z

On Fri, Sep 17, 2021 at 02:34:50PM +0200, Christian König wrote:
> This makes the function much simpler since the complex
> retry logic is now handled else where.
>
> Signed-off-by: Christian König <christian.koenig@amd.com>
> ---
> drivers/dma-buf/dma-resv.c | 86 ++++++++++++++++----------------------
> 1 file changed, 35 insertions(+), 51 deletions(-)
>
> diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c
> index a3c79a99fb44..406150dea5e4 100644
> --- a/drivers/dma-buf/dma-resv.c
> +++ b/drivers/dma-buf/dma-resv.c
> @@ -426,74 +426,58 @@ EXPORT_SYMBOL_GPL(dma_resv_iter_walk);
> */
> 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;
> + struct dma_resv_iter cursor;
> + struct dma_resv_list *list;
> + struct dma_fence *f, *excl;
>
> dma_resv_assert_held(dst);
>
> - rcu_read_lock();
> - src_list = dma_resv_shared_list(src);
> -
> -retry:
> - if (src_list) {
> - unsigned int shared_count = src_list->shared_count;
> -
> - rcu_read_unlock();
> + list = NULL;
> + excl = NULL;
>
> - dst_list = dma_resv_list_alloc(shared_count);
> - if (!dst_list)
> - return -ENOMEM;
> + rcu_read_lock();
> + dma_resv_iter_begin(&cursor, src, true);
> + dma_resv_for_each_fence_unlocked(&cursor, f) {
>
> - rcu_read_lock();
> - src_list = dma_resv_shared_list(src);
> - if (!src_list || src_list->shared_count > shared_count) {
> - kfree(dst_list);
> - goto retry;
> - }
> + if (cursor.is_first) {

Maybe have a wrapper for this, like dma_resv_iter_is_reset or is_first or
is_restart (my preference) with some nice docs that this returns true
everytime we had to restart the sequence?

Otherwise I fully agree, this is so much better with all the hairy
restarting and get_rcu and test_bit shovelled away somewhere.

Either way (but I much prefer a wrapper for is_first):

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

> + dma_resv_list_free(list);
> + dma_fence_put(excl);
>
> - dst_list->shared_count = 0;
> - for (i = 0; i < src_list->shared_count; ++i) {
> - struct dma_fence __rcu **dst;
> - struct dma_fence *fence;
> + if (cursor.fences) {
> + unsigned int cnt = cursor.fences->shared_count;
>
> - fence = rcu_dereference(src_list->shared[i]);
> - if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT,
> - &fence->flags))
> - continue;
> + rcu_read_unlock();
> + list = dma_resv_list_alloc(cnt);
> + if (!list) {
> + dma_resv_iter_end(&cursor);
> + return -ENOMEM;
> + }
>
> - if (!dma_fence_get_rcu(fence)) {
> - dma_resv_list_free(dst_list);
> - src_list = dma_resv_shared_list(src);
> - goto retry;
> - }
> + list->shared_count = 0;
> + rcu_read_lock();
>
> - if (dma_fence_is_signaled(fence)) {
> - dma_fence_put(fence);
> - continue;
> + } else {
> + list = NULL;
> }
> -
> - dst = &dst_list->shared[dst_list->shared_count++];
> - rcu_assign_pointer(*dst, fence);
> + excl = NULL;
> }
> - } else {
> - dst_list = NULL;
> - }
>
> - new = dma_fence_get_rcu_safe(&src->fence_excl);
> + dma_fence_get(f);
> + if (dma_resv_iter_is_exclusive(&cursor))
> + excl = f;
> + else
> + RCU_INIT_POINTER(list->shared[list->shared_count++], f);
> + }
> + dma_resv_iter_end(&cursor);
> rcu_read_unlock();
>
> - src_list = dma_resv_shared_list(dst);
> - old = dma_resv_excl_fence(dst);
> -
> 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);
> + excl = rcu_replace_pointer(dst->fence_excl, excl, dma_resv_held(dst));
> + list = rcu_replace_pointer(dst->fence, list, dma_resv_held(dst));
> write_seqcount_end(&dst->seq);
>
> - dma_resv_list_free(src_list);
> - dma_fence_put(old);
> + dma_resv_list_free(list);
> + dma_fence_put(excl);
>
> return 0;
> }
> --
> 2.25.1
>

--
Daniel Vetter
Software Engineer, Intel Corporation