[PATCH 1/4] misc: Optimize internal usage of __libc_single_threaded

[Florian Weimer]

* Adhemerval Zanella via Libc-alpha:

> diff --git a/elf/libc_early_init.c b/elf/libc_early_init.c
> index 3c4a19cf6b..18966900c4 100644
> --- a/elf/libc_early_init.c
> +++ b/elf/libc_early_init.c
> @@ -16,7 +16,9 @@
>     License along with the GNU C Library; if not, see
>     <https://www.gnu.org/licenses/>.  */
>  
> +#include <assert.h>
>  #include <ctype.h>
> +#include <dlfcn.h>
>  #include <elision-conf.h>
>  #include <libc-early-init.h>
>  #include <libc-internal.h>
> @@ -38,6 +40,13 @@ __libc_early_init (_Bool initial)
>    __libc_single_threaded = initial;
>  
>  #ifdef SHARED
> +  /* _libc_single_thread can be accessed through copy relocations, so it
> +     requires to update the external copy.  */
> +  __libc_external_single_threaded = ___dlsym (RTLD_DEFAULT,
> +					      "__libc_single_threaded");
> +  assert (__libc_external_single_threaded != NULL);
> +  *__libc_external_single_threaded = initial;
> +
>    __libc_initial = initial;
>  #endif

Typo in the comment: [_]_libc_single_thread.

You must use __libc_dlsym, to avoid clobbering dlerror.  No need to add
___dlsym.

Is it necessary to cache the address?

> diff --git a/nptl/pthread_create.c b/nptl/pthread_create.c
> index e7a099acb7..5633d01c62 100644
> --- a/nptl/pthread_create.c
> +++ b/nptl/pthread_create.c
> @@ -627,7 +627,11 @@ __pthread_create_2_1 (pthread_t *newthread, const pthread_attr_t *attr,
>    if (__libc_single_threaded)
>      {
>        late_init ();
> -      __libc_single_threaded = 0;
> +      __libc_single_threaded =
> +#ifdef SHARED
> +        *__libc_external_single_threaded =
> +#endif
> +	0;
>      }
>  
>    const struct pthread_attr *iattr = (struct pthread_attr *) attr;

I think you can call __libc_dlsym here.

Thanks,
Florian