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

["Christian König" <ckoenig.leichtzumerken@gmail.com>]

Am 21.11.2017 um 15:59 schrieb Rob Clark:
> On Tue, Nov 21, 2017 at 9:38 AM, Chris Wilson <chris@chris-wilson.co.uk> 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@gmail.com>
>>> ---
>>>   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
>>>    * 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.

Regards,
Christian.

> BR,
> -R
>
>> -Chris
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