Re: [PATCH] Unify pthread_once (bug 15215)

[Rich Felker <dalias@aerifal.cx>]

On Mon, Aug 26, 2013 at 08:41:50PM +0200, Ondřej Bílka wrote:
> > No, pthread_once _calls_ tend to be once per access to an interface
> > that requires static data to have been initialized, so possibly very
> > often. On the other hand, pthread_once only invokes the init function
> > once per program instance. I don't see anything that would typically
> > happen once per thread, although I suppose you could optimize out
> > calls to pthread_once with tls:
> > 
> Could happen often but dees it? Given need of doing locking you need to
> avoid it in performance critical code. With once per thread I meant an
> patterns:
> 
> computation(){
>   pthread_once(baz,init); // Do common initialization.
>   pthread_create(foo,bar,routine);
> }
> 
> or
> 
> pthread_create(foo,bar,routine);
> 
> with
> 
> routine()
>   {
>     pthread_once(baz,init); // Do common initialization.
>     ...
>   }

These patterns arise is the library is making threads and using
pthread_once to initialize its static data before making the thread.
I'm thinking instead of the case where your library is being _called_
by multi-threaded code, and using pthread_once to ensure that its data
is safely initialized even if there are multiple threads which might
be racing to be the first caller.

> >     static __thread int once_done = 0;
> >     static pthread_once_t once;
> >     if (!once_done) {
> >         pthread_once(&once, init);
> >         once_done = 1;
> >     }
> > 
> > This requires work at the application level, though, and whether it's
> > a net advantage depends a lot on whether multiple threads are likely
> > to be hammering pthread_once on the same once object, and whether the
> > arch has expensive acquire barriers and inexpensive TLS access.
> > 
> Actually you can use following if you are concerned about that use cases:
> 
> #define pthread_once2(x,y) ({   \
>   static __thread int once = 0; \
>   if (!once)                    \
>     pthread_once(x,y);          \
>   once=1;                       \
> })

Indeed; actually, this could even be done in pthread.h, with some
slight variations, perhaps:

#define pthread_once(x,y) ({                  \
  pthread_once_t *__x = (x);                  \
  static __thread pthread_once_t *__once;     \
  if (__once != __x) {                        \
    pthread_once(__x,y);                      \
    __once = __x;                             \
  }                                           \
})

Rich