[PATCH] alsactl: Skip restore during the lock

[Jaroslav Kysela]

Dne 11. 12. 20 v 17:59 Takashi Iwai napsal(a):
> On Fri, 11 Dec 2020 17:45:45 +0100,
> Jaroslav Kysela wrote:
>>
>> Dne 11. 12. 20 v 9:38 Takashi Iwai napsal(a):
>>> Currently alsactl-restore tries to initialize the device when an error
>>> is found for restore action.  But this isn't the right behavior in the
>>> case where the lock is held; it implies that another alsactl is
>>> running concurrently, hence you shouldn't initialize the card at the
>>> same time.  The situation is found easily when two alsactls get
>>> started by both udev and systemd (note that those two invocations are
>>> the designed behavior, see /usr/lib/udev/rules.d/78-sound-cards.rules
>>> for details).
>>>
>>> This patch changes load_state() not to handle the initialization if
>>> the locking fails.
>>
>> The operation should serialize in this case (there's limit of 10 seconds which
>> should be enough to finish the initialization). The state_lock() function
>> should return -EBUSY when the file is locked (and I'm fine to change the
>> behaviour from 'init' to 'skip' for this lock state).
>>
>> It seems that -EEXIST is returned when the lock file exists, but the
>> open(file, O_CREAT|O_EXCL, 0644) caller has not enough priviledges to access
>> this file when another user owns the file.
>>
>> But alsactl from /lib/udev/rules.d/90-alsa-restore.rules and
>> /lib/systemd/system/alsa-restore.service should be run as root, right?
> 
> Yes, it should be root.
> 
> I also wondered how EEXIST comes, too.  Maybe it's also the race
> between the first open(O_RDWR) and the second
> open(O_RDWR|O_CREAT|O_EXCL)?  If so, it'd be better to go back again
> to the normal open(O_RDWR)?

Maybe. It seems enough to add EEXIST errno check to the "if (errno == EBUSY ||
errno == EAGAIN)" condition to repeat the open sequence. The -EBUSY will be
returned correctly then. The one second delay is harmless in my eyes for the
second task.

					Jaroslav

--
Jaroslav Kysela <perex at perex.cz>
Linux Sound Maintainer; ALSA Project; Red Hat, Inc.