[alsa-devel] locking looks odd

[Samuel Thibault]

Jaroslav Kysela, on Wed 17 Aug 2016 19:46:42 +0200, wrote:
> Dne 16.8.2016 v 23:03 Samuel Thibault napsal(a):
> > - 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.
> 
> The thread_safe has this meaning:
> 
> 0  - the pcm plugin is not thread safe
> 1  - the pcm plugin is thread safe (actually only the hw plugin)
> -1 - disable thread safety

So now with rethinking all of this, I'm starting to understand: from
reading the variable name, I would have thought "thread_safe=1" means
"I want thread safety thanks to a mutex", while apparently it means
"the plugin is already thread-safe, there is no need for a mutex"...
Really, all of this should be documented clearly along the source code,
otherwise people will get it wrong.

I'd just like to check something: do we agree that libasound must be
thread-safe by default (otherwise it breaks the application assumption
that it's thread-safe)?  If so, then there are thread-safety bugs: the
mentioned Debian report is far from alone, the upgrade to the newer
libasound has severely broken quite a few applications, I'm at the point
of advising the Debian maintainer to just revert to the previous version
for Stretch, otherwize we'll be shipping just very-buggy software.

Samuel