[alsa-devel] locking looks odd

[Takashi Sakamoto]

Hi,

On Aug 17 2016 06:03, Samuel Thibault wrote:
> Hello,
> 
> We are having odd issues with libasound 1.1.2 which we didn't have with
> libasound 1.1.1, more precisely
> 
> https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=833950
> 
> so I'm having a look at the locking API introduced in 1.1.2, and there
> are some oddities:
> 
> - snd_pcm_new seems to initialize pcm->thread_safe to 0 by default, this
>   does not seem safe. The attached patch initializes it to 1, which
>   fixes the bug in our tests.
> 
> - snd_pcm_hw_open_fd forces it to 1, thus ignoring what snd_pcm_new set.
> 
> - one can find both __snd_pcm_lock and snd_pcm_lock functions, what is
>   the expected difference between them?
> 
> - __snd_pcm_lock takes locks when thread_safe >= 0, while snd_pcm_lock
>   takes locks when thread_safe == 0, this looks really odd.
> 
> - libasound could just not link against libpthread,
>   pthread_mutex_lock/unlock are already provided as empty stubs by libc,
>   the overhead will thus only be hit when the application links against
>   libpthread (libasound will then properly use pthread locks).

Thanks for this information. Unfortunately, committer of this
modification (a subsystem maintainer) has a summer vacation, so it's
delayed for you to get replies for to questions.

In my opinion, the second item you addressed to is a bug of this commit,
to introduce the environment variable.

pcm: Add LIBASOUND_THREAD_SAFE env variable check
http://git.alsa-project.org/?p=alsa-lib.git;a=commitdiff;h=c4b690278eeaf9a6805f67f6709e4aa33e58773a;hp=7a8a1d155267fb6e21ddaa0787d121ec47a15bf2

I don't know exactly the others. Anyway, I'll continue to investigate
these issues and cooperate to solve them.


Thanks

Takashi Sakamoto

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