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

[Adhemerval Zanella <adhemerval dot zanella at linaro dot org>]

On 25/04/2017 18:19, Chris Aoki wrote:
> One of my colleagues suggested that I forward the question
> below to the libc-alpha alias which would increase my chances
> of reaching a glibc malloc expert.   Actually my main question
> (in the original message, below) is a general one, since situations
> calling for pthread_once() can presumably occur in other contexts.
> 
> Chris Aoki
> 
> p.s.  The secondary question, which is specific to glibc malloc, is
> whether ptmalloc_init() can be called concurrently by multiple threads.

It shouldn't, that's why it has the '__malloc_initialized' variable to control
its initialization.  Although, current code without any atomic do not guarantee
very strong semantics (I think it should use something similar to pthread_once
which is indeed __libc_once, see below).

> 
>> Begin forwarded message:
>>
>> From: Chris Aoki <christopher.aoki@oracle.com>
>> Subject: local equivalent for pthread_once() in glibc?
>> Date: April 25, 2017 at 10:50:41 AM PDT
>> To: libc-help@sourceware.org
>> Cc: Chris Aoki <christopher.aoki@oracle.com>
>>
>> I have a question about glibc internals.
>>
>> Is there a private glibc function equivalent to pthread_once()?
>>
>> 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
>>
>> Chris Aoki
> 

This is the default implementation which won't be used on nptl target (basically
all current supported).  It will use the sysdeps/nptl/libc-lockP.h in fact:

250 /* Call handler iff the first call.  */
251 #define __libc_once(ONCE_CONTROL, INIT_FUNCTION) \
252   do {                                                                        \
253     if (PTFAVAIL (__pthread_once))                                            \
254       __libc_ptf_call_always (__pthread_once, (&(ONCE_CONTROL),               \
255                                                INIT_FUNCTION));               \
256     else if ((ONCE_CONTROL) == PTHREAD_ONCE_INIT) {                           \
257       INIT_FUNCTION ();                                                       \
258       (ONCE_CONTROL) |= 2;                                                    \
259     }                                                                         \
260   } while (0)


Which will call pthread_once for multithread programs (since PTFAVAIL will
returns true).