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

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Date:   Tue, 21 Nov 2017 09:59:19 -0500
From:   Rob Clark <robdclark@...il.com>
To:     Chris Wilson <chris@...is-wilson.co.uk>
Cc:     "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "linaro-mm-sig@...ts.linaro.org" <linaro-mm-sig@...ts.linaro.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "linux-media@...r.kernel.org" <linux-media@...r.kernel.org>
Subject: Re: [PATCH] reservation: don't wait when timeout=0

On Tue, Nov 21, 2017 at 9:38 AM, Chris Wilson <chris@...is-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@...il.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..

BR,
-R

> -Chris