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

From: Christian KÃnig
Date: Tue Nov 21 2017 - 10:50:05 EST

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.



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