[alsa-devel] [PATCH 09/15] ALSA: line6: Add hwdep interface to access the POD control messages

[Andrej Kruták]

On Fri, Aug 12, 2016 at 2:30 PM, Takashi Iwai <tiwai at suse.de> wrote:
> On Fri, 12 Aug 2016 14:15:16 +0200,
>>
>> >> > Also, the blocking read/write control isn't usually done by a
>> >> > semaphore.  Then you can handle the interrupt there.
>> >> >
>> >> >
>> >>
>> >> I actually wonder why, semaphores seemed perfect for this... Do you
>> >> have some hints?
>> >
>> > Assume you want to interrupt the user-space app while it's being
>> > blocked by the semaphore.  With your code, you can't.
>> >
>> >
>>
>> You can - down_interruptible() is there for this exact reason.
>
> There is another blocking path: you keep semaphore down after
> line6_hwdep_read() until line6_hwdep_push_message().  What happens if
> user-space interrupts during that, and line6_hwdep_push_message() is
> delayed or stall by some reason?
>

Actually, I think I don't see what's the "another path" here, could
you please elaborate one more bit? I just want to make sure to not
reimplement the same race using waitqueue...

What's the point then? line6_hwdep_push_message() could get not
scheduled for some while. So until it calls up(), line6_hwdep_read()
will block on down_interruptible(), or until signal (in which case
user gets -ERESTARTSYS). After up() is called, there are data in
buffer... If line6_hwdep_read() returns after interrupt, no problem -
the buffer will just continue to be filled and semaphore will be
up()'d while there's free buffer space. Or until the device is
closed...

If I do the same via waitqueue, I will have the same problems, no?
Maybe if you could post the steps where you see the race...

At the same time, looking at __down_common(), it just does the
standard waitqueue stuff (TASK_INTERRUPTIBLE + schedule()) (+counter
in down())... So do you have some other race in mind? I'm running in
circles, so surely you must :-)


Sorry if I sound like a moron... and thanks for you time!

-- 
Andrej