[pulseaudio-discuss] pa_once can run twice?

David Henningsson david.henningsson at canonical.com
Tue Apr 24 06:01:15 PDT 2012

On 04/23/2012 05:03 PM, Dalleau, Frederic wrote:
> Hi David,
> On Fri, Apr 20, 2012 at 4:44 PM, David Henningsson
> <david.henningsson at canonical.com>  wrote:
>> While researching a bug I came across something that might be a bug in the
>> pa_once logic, but this stuff is tricky, so I might also be missing
>> something.
>> Imagine this:
>>   * Thread 1 runs pa_once_begin, succeeds and starts running the payload (i e
>> the code that should only run once).
>>   * Thread 2 starts running pa_once_begin, but only the first row. We're now
>> right *before* pa_atomic_inc(&control->ref) but *after*
>> pa_atomic_load(&control->done).
>>   * Thread 1 finishes the payload, runs pa_once_done which sets control->done
>> and frees the mutex.
>>   * Thread 2 continues, pa_once_begin succeeds and the payload is now run a
>> second time!
> After reading your mail, I made some experiments by adding a usleep() call
> in Thread 1 between pa_atomic_load(&control->done) and
> pa_atomic_inc(&control->ref)
> and that failed once-test 100% of time.
> I reverted the usleep and made another experiment using 50000 iterations in
> once-test and it just failed.
> Good catch !

I tried to look up implementation/algorithm suggestions, and for the 
ones I found [1], there was no freeing of the mutex. Without freeing, 
the code becomes simpler. The attached patch is a version of that. I've 
just tried a simple once-test (but do feel free to run it 50000 times :-) ).

But of course, now we leak a mutex. But that's what we already do with 
the static mutexes we use in a few places already, so maybe it doesn't 
matter much?

David Henningsson, Canonical Ltd.

[1] e g http://en.wikipedia.org/wiki/Double-checked_locking

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