[FFmpeg-devel] [PATCH] w32pthreads: add pthread_once emulation

[Hendrik Leppkes]

On Wed, Oct 7, 2015 at 6:29 PM, Hendrik Leppkes <h.leppkes at gmail.com> wrote:
> On Wed, Oct 7, 2015 at 6:23 PM, Matt Oliver <protogonoi at gmail.com> wrote:
>> On 6 October 2015 at 21:36, Hendrik Leppkes <h.leppkes at gmail.com> wrote:
>>
>>> The emulation uses native InitOnce* APIs on Windows Vista+, and a
>>> lock-free/allocation-free approach using atomics and spinning for Windows
>>> XP.
>>> ---
>>>
>>> This is in preparation to use pthread_once for global static init
>>> functions,
>>> and eventually removing the global lock in avcodec_open2
>>>
>>> compat/w32pthreads.h | 68
>>> ++++++++++++++++++++++++++++++++++++++++++++++++++++
>>>  1 file changed, 68 insertions(+)
>>>
>>> diff --git a/compat/w32pthreads.h b/compat/w32pthreads.h
>>> index deb1c53..8523976 100644
>>> --- a/compat/w32pthreads.h
>>> +++ b/compat/w32pthreads.h
>>> @@ -154,6 +154,19 @@ static inline int pthread_cond_signal(pthread_cond_t
>>> *cond)
>>>      return 0;
>>>  }
>>>
>>> +typedef INIT_ONCE pthread_once_t;
>>> +#define PTHREAD_ONCE_INIT INIT_ONCE_STATIC_INIT
>>> +
>>> +static av_unused int pthread_once(pthread_once_t *once_control, void
>>> (*init_routine)(void))
>>> +{
>>> +    BOOL pending = FALSE;
>>> +    InitOnceBeginInitialize(once_control, 0, &pending, NULL);
>>> +    if (pending)
>>> +        init_routine();
>>> +    InitOnceComplete(once_control, 0, NULL);
>>> +    return 0;
>>> +}
>>> +
>>>  #else // _WIN32_WINNT < 0x0600
>>>  /* for pre-Windows 6.0 platforms we need to define and use our own
>>> condition
>>>   * variable and api */
>>> @@ -304,6 +317,57 @@ static av_unused int
>>> pthread_cond_signal(pthread_cond_t *cond)
>>>      pthread_mutex_unlock(&win32_cond->mtx_broadcast);
>>>      return 0;
>>>  }
>>> +
>>> +/* for pre-Windows 6.0 platforms, define INIT_ONCE struct,
>>> + * compatible to the one used in the native API */
>>> +
>>> +typedef union pthread_once_t  {
>>> +    void * Ptr;    ///< For the Windows 6.0+ native functions
>>> +    LONG state;    ///< For the pre-Windows 6.0 compat code
>>> +} pthread_once_t;
>>> +
>>> +#define PTHREAD_ONCE_INIT {0}
>>> +
>>> +/* function pointers to init once API on windows 6.0+ kernels */
>>> +static BOOL (WINAPI *initonce_begin)(pthread_once_t *lpInitOnce, DWORD
>>> dwFlags, BOOL *fPending, void **lpContext);
>>> +static BOOL (WINAPI *initonce_complete)(pthread_once_t *lpInitOnce, DWORD
>>> dwFlags, void *lpContext);
>>> +
>>> +static av_unused int pthread_once(pthread_once_t *once_control, void
>>> (*init_routine)(void))
>>> +{
>>> +    /* Use native functions on Windows 6.0+ */
>>> +    if (initonce_begin && initonce_complete)
>>> +    {
>>> +        BOOL pending = FALSE;
>>> +        initonce_begin(once_control, 0, &pending, NULL);
>>> +        if (pending)
>>> +            init_routine();
>>> +        initonce_complete(once_control, 0, NULL);
>>> +        return 0;
>>> +    }
>>> +
>>> +    /* pre-Windows 6.0 compat using a spin-lock */
>>> +    switch (InterlockedCompareExchange(&once_control->state, 1, 0))
>>> +    {
>>> +        /* Initial run */
>>> +        case 0:
>>> +            init_routine();
>>> +            InterlockedExchange(&once_control->state, 2);
>>> +            break;
>>> +        /* Another thread is running init */
>>> +        case 1:
>>> +            while (1) {
>>> +                MemoryBarrier();
>>> +                if (once_control->state == 2)
>>> +                    break;
>>> +                Sleep(0);
>>> +            }
>>> +            break;
>>> +        /* Initialization complete */
>>> +        case 2:
>>> +            break;
>>> +    }
>>> +    return 0;
>>> +}
>>>  #endif
>>>
>>>  static av_unused void w32thread_init(void)
>>> @@ -319,6 +383,10 @@ static av_unused void w32thread_init(void)
>>>          (void*)GetProcAddress(kernel_dll, "WakeConditionVariable");
>>>      cond_wait      =
>>>          (void*)GetProcAddress(kernel_dll, "SleepConditionVariableCS");
>>> +    initonce_begin =
>>> +        (void*)GetProcAddress(kernel_dll, "InitOnceBeginInitialize");
>>> +    initonce_complete =
>>> +        (void*)GetProcAddress(kernel_dll, "InitOnceComplete");
>>>  #endif
>>>
>>>  }
>>> --
>>> 2.5.3.windows.1
>>
>>
>> LGTM
>
> There is a new version of the patch which also solves the
> w32thread_init mess, so this one isn't the final version anymore.
> Current working version is here:
> https://github.com/Nevcairiel/FFmpeg/commits/pthread_once
>

This version has been applied.