Re: Fwd: local equivalent for pthread_once() in glibc?

[Florian Weimer <fweimer at redhat dot com>]

On 04/25/2017 11:19 PM, Chris Aoki wrote:

> p.s.  The secondary question, which is specific to glibc malloc, is
> whether ptmalloc_init() can be called concurrently by multiple threads.

No, but the reason is really subtle: pthread_create calls calloc before 
creating the new thread. :)

#0  ptmalloc_init () at arena.c:255
#1  0x00007ffff787a7ad in ptmalloc_init () at malloc.c:2935
#2  malloc_hook_ini (sz=272, caller=<optimized out>) at hooks.c:31
#3  0x00007ffff7879d8a in __libc_calloc (n=<optimized out>,
    elem_size=<optimized out>) at malloc.c:3234
#4  0x00007ffff7deac2b in allocate_dtv (result=0x7ffff77f2700) at 
dl-tls.c:322
#5  __GI__dl_allocate_tls (mem=mem@entry=0x7ffff77f2700) at dl-tls.c:539
#6  0x00007ffff7bc128a in allocate_stack (stack=<synthetic pointer>,
    pdp=<synthetic pointer>, attr=0x7fffffffe520) at allocatestack.c:586
#7  __pthread_create_2_1 (newthread=0x7fffffffe5a8, attr=0x0,
    start_routine=0x4005c6 <f>, arg=0x0) at pthread_create.c:539

> > I have a structure that is frequently accessed after initialization
> > so putting a lock around initialization and a check for initialization
> > would add considerable overhead.   Normally one would use pthread_once()
> > in this situation, but my colleagues tell me that adding a reference to an
> > external function is prohibited within libc.so.   I see a macro __libc_once()
> > but it does not appear to synchronize:
> >
> > /* Define once control variable.  */
> > #define __libc_once_define(CLASS, NAME) CLASS int NAME = 0
> >
> > /* Call handler iff the first call.  */
> > #define __libc_once(ONCE_CONTROL, INIT_FUNCTION) \
> >   do {                                                                        \
> >     if ((ONCE_CONTROL) == 0) {                                                \
> >       INIT_FUNCTION ();                                                       \
> >       (ONCE_CONTROL) = 1;                                                     \
> >     }                                                                         \
> >   } while (0)
> >
> > Any clues appreciated.  Thanks

There is an override for the generic definition in 
sysdeps/nptl/libc-lockP.h, which is used on Linux.  In general, you have 
to examine the source tree carefully to discover such overrides.

Thanks,
Florian