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

[Takashi Sakamoto <o-takashi@sakamocchi.jp>]

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