Re: Why doesn't mixer control (values) have some kind of locking mechanism? (mutex?)

[Tom Yan <tom.ty89@gmail.com>]

Yeah I suppose a "full" lock would do. (That was what I was trying to
point out. I don't really understand Pierre's message. I merely
suppose you need some facility in the kernel anyway so that you can
lock from userspace.) I hope that amixer the utility will at least
have the capability to reschedule/wait by then though (instead of just
"failing" like in your python demo).

As for the compare-and-swap part, it's just a plus. Not that
"double-looping" for *each* channel doesn't work. It just again seems
silly and primitive (and was once confusing to me).

On Thu, 6 Aug 2020 at 17:15, Takashi Sakamoto <o-takashi@sakamocchi.jp> wrote:
>
> Hi,
>
> On Thu, Aug 06, 2020 at 04:57:02PM +0800, Tom Yan wrote:
> > The problem/race I am trying to point out is, one process can
> > get()/read before another finishing its get()+put() pair (which is
> > required by volume setting/adjusting), so something like
> > get1()->get2()->put1()->put2() could easily happen (where each put()
> > relies on / is "configured" with volumes of their respective get()).
> > The lock will need to intentionally stall further get()/read as well.
>
> In my opinion, in the above case, it's possible to serialize each
> transaction which consists of get/put (read/write in userspace
> application) with lock/unlock mechanism.
>
> +-----------+-----------+
> | process A | process B |
> +-----------+-----------+
> |   lock    |           |
> |   get     |           |
> |           |lock(EPERM)| reschedule lock/get/set/unlock actions
> |   set     |           |
> |           |lock(EPERM)| reschedule lock/get/set/unlock actions
> |  unlock   |           |
> |           |   lock    |
> |           |   get     |
> |           |   set     |
> |           |  unlock   |
> +-----------+-----------+
>
> (Of course, the above is achieved when the series of operations is done
> by userspace applications. For simplicity, I don't mention about
> in-kernel initiators of the get/set actions. In this point, I don't
> address to the message Pierre posted.)
>
> > If we for some reason want to avoid using locks, put() needs to be
> > atomic by design (like, "embed" get() in itself and use arrays for
> > volume values, instead of requiring those to be implemented in the
> > userspace manually / with a loop). Unfortunately that isn't the case
> > in ALSA.
>
> I get your intension is something like compare-and-swap[1]. At present,
> ALSA control core has no functionality like it, but it's worth to
> investigate. The ioctl request should includes a pair of 'struct
> snd_ctl_elem_value' in its argument. In a design of ALSA control
> core, the pair should be processed in each driver since ALSA control
> core has no 'cache' of the content of 'struct snd_ctl_elem_value' except
> for user-defined control element set.
>
> Here, would I ask your opinion to the lock/get/set/unlock actions
> from userspace? It can really not be one of solution for the issue?
>
> [1] https://en.wikipedia.org/wiki/Compare-and-swap
>
>
> Regards
>
> Takashi Sakamoto