Re: [PATCH] reservation: don't wait when timeout=0

From: Christian KÃnig
Date: Tue Nov 21 2017 - 11:11:45 EST

Am 21.11.2017 um 16:58 schrieb Chris Wilson:
Quoting Christian KÃnig (2017-11-21 15:49:55)
Am 21.11.2017 um 15:59 schrieb Rob Clark:
On Tue, Nov 21, 2017 at 9:38 AM, Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> wrote:
Quoting Rob Clark (2017-11-21 14:08:46)
If we are testing if a reservation object's fences have been
signaled with timeout=0 (non-blocking), we need to pass 0 for
timeout to dma_fence_wait_timeout().

Plus bonus spelling correction.

Signed-off-by: Rob Clark <robdclark@xxxxxxxxx>
drivers/dma-buf/reservation.c | 11 +++++++++--
1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/drivers/dma-buf/reservation.c b/drivers/dma-buf/reservation.c
index dec3a815455d..71f51140a9ad 100644
--- a/drivers/dma-buf/reservation.c
+++ b/drivers/dma-buf/reservation.c
@@ -420,7 +420,7 @@ EXPORT_SYMBOL_GPL(reservation_object_get_fences_rcu);
* Returns -ERESTARTSYS if interrupted, 0 if the wait timed out, or
- * greater than zer on success.
+ * greater than zero on success.
long reservation_object_wait_timeout_rcu(struct reservation_object *obj,
bool wait_all, bool intr,
@@ -483,7 +483,14 @@ long reservation_object_wait_timeout_rcu(struct reservation_object *obj,
goto retry;

- ret = dma_fence_wait_timeout(fence, intr, ret);
+ /*
+ * Note that dma_fence_wait_timeout() will return 1 if
+ * the fence is already signaled, so in the wait_all
+ * case when we go through the retry loop again, ret
+ * will be greater than 0 and we don't want this to
+ * cause _wait_timeout() to block
+ */
+ ret = dma_fence_wait_timeout(fence, intr, timeout ? ret : 0);
One should ask if we should just fix the interface to stop returning
incorrect results (stop "correcting" a completion with 0 jiffies remaining
as 1). A timeout can be distinguished by -ETIME (or your pick of errno).
perhaps -EBUSY, if we go that route (although maybe it should be a
follow-on patch, this one is suitable for backport to stable/lts if
one should so choose..)

I think current approach was chosen to match schedule_timeout() and
other such functions that take a timeout in jiffies. Not making a
judgement on whether that is a good or bad reason..
We intentionally switched away from that to be in sync with the
wait_event_* interface.

Returning 1 when a function with a zero timeout succeeds is actually
quite common in the kernel.
We actually had this conversation last time, and outside of that it
isn't. Looking at all the convolution to first return 1, then undo the
damage in the caller, it looks pretty silly.

I don't find that very intuitive either, but you would also have to handle the error code in the calling function as well.

And it is what the whole kernel does all over the place with it's wait_event_* and scheduler timeouts as well.