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

[Christian KÃnig]

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
> > >    * 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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