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SubjectRe: complete_all and "forever" completions
On Wed, Oct 26, 2016 at 05:42:13PM +0200, Peter Zijlstra wrote:
> On Wed, Oct 26, 2016 at 05:10:01AM -0700, Dmitry Torokhov wrote:
> > On Wed, Oct 26, 2016 at 10:45:35AM +0200, Peter Zijlstra wrote:
> > > On Tue, Oct 25, 2016 at 03:30:54PM -0700, Dmitry Torokhov wrote:
>
> > > > Or do we need something like this in
> > > > do_wait_for_common():
> > > >
> > > > if (x->done < UINT_MAX/2)
> > > > x->done--;
> > >
> > > Depends a bit, do you really want this? Seems a bit daft to keep asking
> > > if its done already, seems like a waste of cycles to me.
> > >
> >
> > The use case I am after is:
> >
> > 1. There is a device that is extremely dumb without firmware
> > 2. The driver uses request_firmware_nowait() and signals completion from
> > the firmware loading callback to let the reset of the driver know that
> > firmware has been done loading (successfully or otherwise)
> > 3. The driver uses wait_for_completion() in both remove() and suspend()
> > methods to wait for the firmware to finish loading.
> >
> > While remove() happens at most once per device instance, suspend() may
> > happen unbound number of times (theoretically).
> >
> > So the question is: should complete_all have this "forever" semantic
> > (IOW is documentation right about the intent) or do we need a new
> > primitive for this? From the cursory glance of users of complete_all()
> > all of them expect completion to stay in signalled state either forever,
> > or until they call reinit_completion() explicitly.
>
> Nah, if we need this we should fix this one. Adding similar but slightly
> different primitives is a pain.
>
> But I think you might need slightly more than the proposed change, the
> case I worry about is doing complete_all() when done != 0 (which isn't
> all that strange).
>
>
> Does something like so work?

Yes, this looks good to me.

>
> ---
> kernel/sched/completion.c | 7 +++++--
> 1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/sched/completion.c b/kernel/sched/completion.c
> index 8d0f35debf35..5deab9c789df 100644
> --- a/kernel/sched/completion.c
> +++ b/kernel/sched/completion.c
> @@ -51,7 +51,7 @@ void complete_all(struct completion *x)
> unsigned long flags;
>
> spin_lock_irqsave(&x->wait.lock, flags);
> - x->done += UINT_MAX/2;
> + x->done = UINT_MAX/2;
> __wake_up_locked(&x->wait, TASK_NORMAL, 0);
> spin_unlock_irqrestore(&x->wait.lock, flags);
> }
> @@ -79,7 +79,10 @@ do_wait_for_common(struct completion *x,
> if (!x->done)
> return timeout;
> }
> - x->done--;
> +
> + if (x->done != UINT_MAX/2)
> + x->done--;
> +
> return timeout ?: 1;
> }
>

--
Dmitry

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