On Mon, Aug 29, 2016 at 08:08:33AM +0100, Chris Wilson wrote:
> With the seqlock now extended to cover the lookup of the fence and its
> testing, we can perform that testing solely under the seqlock guard and
> avoid the effective locking and serialisation of acquiring a reference to
> the request. As the fence is RCU protected we know it cannot disappear
> as we test it, the same guarantee that made it safe to acquire the
> reference previously. The seqlock tests whether the fence was replaced
> as we are testing it telling us whether or not we can trust the result
> (if not, we just repeat the test until stable).
>
> Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> Cc: Sumit Semwal <sumit.semwal@xxxxxxxxxx>
> Cc: linux-media@xxxxxxxxxxxxxxx
> Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx
> Cc: linaro-mm-sig@xxxxxxxxxxxxxxxx
Not entirely sure this is safe for non-i915 drivers. We might now call
->signalled on a zombie fence (i.e. refcount == 0, but not yet kfreed).
i915 can do that, but other drivers might go boom.
I think in generic code we can't do these kind of tricks unfortunately.
-Daniel
> ---
> drivers/dma-buf/reservation.c | 32 ++++----------------------------
> 1 file changed, 4 insertions(+), 28 deletions(-)
>
> diff --git a/drivers/dma-buf/reservation.c b/drivers/dma-buf/reservation.c
> index e74493e7332b..1ddffa5adb5a 100644
> --- a/drivers/dma-buf/reservation.c
> +++ b/drivers/dma-buf/reservation.c
> @@ -442,24 +442,6 @@ unlock_retry:
> }
> EXPORT_SYMBOL_GPL(reservation_object_wait_timeout_rcu);
>
> -
> -static inline int
> -reservation_object_test_signaled_single(struct fence *passed_fence)
> -{
> - struct fence *fence, *lfence = passed_fence;
> - int ret = 1;
> -
> - if (!test_bit(FENCE_FLAG_SIGNALED_BIT, &lfence->flags)) {
> - fence = fence_get_rcu(lfence);
> - if (!fence)
> - return -1;
> -
> - ret = !!fence_is_signaled(fence);
> - fence_put(fence);
> - }
> - return ret;
> -}
> -
> /**
> * reservation_object_test_signaled_rcu - Test if a reservation object's
> * fences have been signaled.
> @@ -474,7 +456,7 @@ bool reservation_object_test_signaled_rcu(struct reservation_object *obj,
> bool test_all)
> {
> unsigned seq, shared_count;
> - int ret;
> + bool ret;
>
> rcu_read_lock();
> retry:
> @@ -494,10 +476,8 @@ retry:
> for (i = 0; i < shared_count; ++i) {
> struct fence *fence = rcu_dereference(fobj->shared[i]);
>
> - ret = reservation_object_test_signaled_single(fence);
> - if (ret < 0)
> - goto retry;
> - else if (!ret)
> + ret = fence_is_signaled(fence);
> + if (!ret)
> break;
> }
>
> @@ -509,11 +489,7 @@ retry:
> struct fence *fence_excl = rcu_dereference(obj->fence_excl);
>
> if (fence_excl) {
> - ret = reservation_object_test_signaled_single(
> - fence_excl);
> - if (ret < 0)
> - goto retry;
> -
> + ret = fence_is_signaled(fence_excl);
> if (read_seqcount_retry(&obj->seq, seq))
> goto retry;
> }
> --
> 2.9.3
>
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--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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