Re: [Intel-gfx] [PATCH 05/26] dma-buf: use new iterator in dma_resv_wait_timeout

Daniel Vetter
2021-09-17T14:43:32Z

On Fri, Sep 17, 2021 at 02:34:52PM +0200, Christian König wrote:
> This makes the function much simpler since the complex
> retry logic is now handled elsewhere.
>
> Signed-off-by: Christian König <christian.koenig@amd.com>
> ---
> drivers/dma-buf/dma-resv.c | 68 ++++++--------------------------------
> 1 file changed, 10 insertions(+), 58 deletions(-)
>
> diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c
> index 9b90bd9ac018..c7db553ab115 100644
> --- a/drivers/dma-buf/dma-resv.c
> +++ b/drivers/dma-buf/dma-resv.c
> @@ -569,74 +569,26 @@ long dma_resv_wait_timeout(struct dma_resv *obj, bool wait_all, bool intr,
> unsigned long timeout)
> {
> long ret = timeout ? timeout : 1;
> - unsigned int seq, shared_count;
> + struct dma_resv_iter cursor;
> struct dma_fence *fence;
> - int i;
>
> -retry:
> - shared_count = 0;
> - seq = read_seqcount_begin(&obj->seq);
> rcu_read_lock();

I missed this in my previous conversion reviews, but pls move the
rcu_read_lock into the iterator. That should simplify the flow in all of
these quite a bit more, and since the iter_next_unlocked grabs a full
reference for the iteration body we really don't need that protected by
rcu.

We can't toss rcu protection for dma_resv anytime soon (if ever), but we
can at least make it an implementation detail.

> - i = -1;
> -
> - fence = dma_resv_excl_fence(obj);
> - if (fence && !test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags)) {
> - if (!dma_fence_get_rcu(fence))
> - goto unlock_retry;
> + dma_resv_iter_begin(&cursor, obj, wait_all);
> + dma_resv_for_each_fence_unlocked(&cursor, fence) {
> + rcu_read_unlock();
>
> - if (dma_fence_is_signaled(fence)) {
> - dma_fence_put(fence);
> - fence = NULL;
> + ret = dma_fence_wait_timeout(fence, intr, ret);
> + if (ret <= 0) {
> + dma_resv_iter_end(&cursor);
> + return ret;
> }
>
> - } else {
> - fence = NULL;
> - }
> -
> - if (wait_all) {
> - struct dma_resv_list *fobj = dma_resv_shared_list(obj);
> -
> - if (fobj)
> - shared_count = fobj->shared_count;
> -
> - for (i = 0; !fence && i < shared_count; ++i) {
> - struct dma_fence *lfence;
> -
> - lfence = rcu_dereference(fobj->shared[i]);
> - if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT,
> - &lfence->flags))
> - continue;
> -
> - if (!dma_fence_get_rcu(lfence))
> - goto unlock_retry;
> -
> - if (dma_fence_is_signaled(lfence)) {
> - dma_fence_put(lfence);
> - continue;
> - }
> -
> - fence = lfence;
> - break;
> - }
> + rcu_read_lock();
> }
> -
> + dma_resv_iter_end(&cursor);
> 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))
> - goto retry;
> - }
> return ret;
> -
> -unlock_retry:
> - rcu_read_unlock();
> - goto retry;

I think we still have the same semantics, and it's so much tidier.

With the rcu_read_unlock stuff into iterators (also applies to previous
two patches):

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

> }
> EXPORT_SYMBOL_GPL(dma_resv_wait_timeout);
>
> --
> 2.25.1
>

--
Daniel Vetter
Software Engineer, Intel Corporation