[Bug libgcc/87189] New: libgcc/gthr-posix.h (__gthread_active_p) makes unwarranted assumptions about libpthread.a

[ppluzhnikov at google dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87189

            Bug ID: 87189
           Summary: libgcc/gthr-posix.h (__gthread_active_p) makes
                    unwarranted assumptions about libpthread.a
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libgcc
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ppluzhnikov at google dot com
  Target Milestone: ---

Redirected here from GLIBC bug:
https://sourceware.org/bugzilla/show_bug.cgi?id=21777

A trivial program using pthread_key_create but not pthread_mutex_lock will
crash on GLIBC, when linked statically.

Current code in libgcc/gthr-posix.h:

  #ifdef __GLIBC__
  __gthrw2(__gthrw_(__pthread_key_create),
           __pthread_key_create,
           pthread_key_create)
  # define GTHR_ACTIVE_PROXY      __gthrw_(__pthread_key_create)

assumes that if __pthread_key_create is linked in, then
pthread_mutex_{lock,unlock} will also be (__gthread_active_p() returns 1).

As attached test demonstrates, that is not necessarily the case.
Workaround: add -Wl,-u,pthread_mutex_lock -Wl,-u,pthread_mutex_unlock to the
link line.

Confirmed with current GLIBC trunk (a6e8926f8d49a213a9abb1a61f6af964f612ab7f)
and GCC @264043.


P.S. Why would a program use pthread_key_create but not pthread_mutex_lock?

Suppose you have a piece of data you want to memoize between calls to a certain
function, and that the data needs to be modifiable. It's convenient to make
that data thread-local, so the function is both thread-safe and parallelizable.


--- test.c ---

/* Link with "gcc -pthread test.c -static"  */
#include <pthread.h>

pthread_key_t k;

int
main (int argc, char *argv[])
{
  pthread_key_create (&k, NULL);
  pthread_setspecific (k, NULL);

  return 0;
}