Re: Races in alsa-lib with threads

[Jaroslav Kysela <perex@perex.cz>]

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@suse.de] Sent: Tuesday, November 13,
>>> 2012 2:30 AM To: Trent Piepho Cc: Krakora Robert-B42341;
>>> alsa-devel@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@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@perex.cz>
Linux Kernel Sound Maintainer
ALSA Project; Red Hat, Inc.