[alsa-devel] Races in alsa-lib with threads

[Krakora Robert-B42341]

From: Jaroslav Kysela [perex at perex.cz]
Sent: Tuesday, November 13, 2012 9:08 AM
To: Takashi Iwai
Cc: Krakora Robert-B42341; alsa-devel at alsa-project.org; Trent Piepho
Subject: Re: [alsa-devel] Races in alsa-lib with threads

Date 13.11.2012 14:50, Takashi Iwai wrote:
> At Tue, 13 Nov 2012 13:39:24 +0000, Krakora Robert-B42341 wrote:
>>
>> [1  <text/plain; us-ascii (quoted-printable)>]
>>> From: Takashi Iwai [tiwai at suse.de] Sent: Tuesday, November 13,
>>> 2012 2:30 AM To: Trent Piepho Cc: Krakora Robert-B42341;
>>> alsa-devel at alsa-project.org Subject: Re: [alsa-devel] Races in
>>> alsa-lib with threads
>>>
>>> At Tue, 13 Nov 2012 02:14:08 -0500, Trent Piepho wrote:
>>>>
>>>> On Tue, Nov 13, 2012 at 1:32 AM, Takashi Iwai <tiwai at suse.de>
>>>> wrote:
>>>>> At Mon, 12 Nov 2012 19:40:42 +0000 (UTC), Rob Krakora wrote:
>>>>>> Would you be able to point me the the ALSA documentation
>>>>>> that indicates the stipulations on handle usage using
>>>>>> multiple threads?  I cannot find it.
>>>>>
>>>>> Think other way round: The fact that it isn't documented
>>>>> means it's not safe to use in that way :)
>>>>
>>>>
>>>> The introduction on the alsa-project home page says, "SMP and
>>>> thread-safe design. "  Some people might misunderstand what
>>>> thread-safe means.  Maybe some clarification could be added.
>>>> "Different streams are SMP and thread-safe (calls for the same
>>>> stream must be serialized)."
>>>
>>> Yeah this should be corrected.  SMP things are just about the
>>> kernel side at that time.
>>>
>>> It's a Wiki, feel free to correct / improve the text.
>>>
>>>
>>> Takashi
>>
>> Hi Takashi,
>>
>> The way that the GStreamer ALSA Sink Plugin is using ALSA Lib
>> assumes that it is thread safe.  The fix crafted by one of Trent's
>> colleagues (attached) seems to be the way to go.  However, I don't
>> know how it would impact other functionality within ALSA Lib.
>
> No, sorry, we don't want to introduce pthread_lock in alsa-lib PCM
> stuff.
>
> We already have a few places, yes, but they are exceptional (mostly
> for funky rarely used plugins that we can drop or mixer codes that
> are no hot path).

I tried to put notes on the ALSA Wiki, please, extend / fix / review.

http://www.alsa-project.org/main/index.php/SMP_Design

                                        Jaroslav

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

Hi Jaroslav,

Looks good, except I think the text in the link on the main page should be changed from "see these notes" to "PLEASE READ FIRST" in all caps for emphasis.

Best Regards,

Rob Krakora