/* Copyright (C) 2003-2018 Free Software Foundation, Inc.

   This file is part of the GNU C Library.

   Contributed by Jakub Jelinek <jakub@redhat.com>, 2003.

   The GNU C Library is free software; you can redistribute it and/or

   modify it under the terms of the GNU Lesser General Public

   License as published by the Free Software Foundation; either

   version 2.1 of the License, or (at your option) any later version.

   The GNU C Library is distributed in the hope that it will be useful,

   but WITHOUT ANY WARRANTY; without even the implied warranty of

   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.	 See the GNU

   Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public

   License along with the GNU C Library; if not, see

   <http://www.gnu.org/licenses/>.  */

#include "pthreadP.h"

#include <futex-internal.h>

#include <atomic.h>

unsigned long int __fork_generation attribute_hidden;

static void

clear_once_control (void *arg)

{

  pthread_once_t *once_control = (pthread_once_t *) arg;

  /* Reset to the uninitialized state here.  We don't need a stronger memory

     order because we do not need to make any other of our writes visible to

     other threads that see this value: This function will be called if we

     get interrupted (see __pthread_once), so all we need to relay to other

     threads is the state being reset again.  */

  atomic_store_relaxed (once_control, 0);

  futex_wake ((unsigned int *) once_control, INT_MAX, FUTEX_PRIVATE);

}

/* This is similar to a lock implementation, but we distinguish between three

   states: not yet initialized (0), initialization in progress

   (__fork_generation | __PTHREAD_ONCE_INPROGRESS), and initialization

   finished (__PTHREAD_ONCE_DONE); __fork_generation does not use the bits

   that are used for __PTHREAD_ONCE_INPROGRESS and __PTHREAD_ONCE_DONE (which

   is what __PTHREAD_ONCE_FORK_GEN_INCR is used for).  If in the first state,

   threads will try to run the initialization by moving to the second state;

   the first thread to do so via a CAS on once_control runs init_routine,

   other threads block.

   When forking the process, some threads can be interrupted during the second

   state; they won't be present in the forked child, so we need to restart

   initialization in the child.  To distinguish an in-progress initialization

   from an interrupted initialization (in which case we need to reclaim the

   lock), we look at the fork generation that's part of the second state: We

   can reclaim iff it differs from the current fork generation.

   XXX: This algorithm has an ABA issue on the fork generation: If an

   initialization is interrupted, we then fork 2^30 times (30 bits of

   once_control are used for the fork generation), and try to initialize

   again, we can deadlock because we can't distinguish the in-progress and

   interrupted cases anymore.

   XXX: We split out this slow path because current compilers do not generate

   as efficient code when the fast path in __pthread_once below is not in a

   separate function.  */

static int

__attribute__ ((noinline))

__pthread_once_slow (pthread_once_t *once_control, void (*init_routine) (void))

{

  while (1)

    {

      int val, newval;

      /* We need acquire memory order for this load because if the value

         signals that initialization has finished, we need to see any

         data modifications done during initialization.  */

      val = atomic_load_acquire (once_control);

      do

	{

	  /* Check if the initialization has already been done.  */

	  if (__glibc_likely ((val & __PTHREAD_ONCE_DONE) != 0))

	    return 0;

	  /* We try to set the state to in-progress and having the current

	     fork generation.  We don't need atomic accesses for the fork

	     generation because it's immutable in a particular process, and

	     forked child processes start with a single thread that modified

	     the generation.  */

	  newval = __fork_generation | __PTHREAD_ONCE_INPROGRESS;

	  /* We need acquire memory order here for the same reason as for the

	     load from once_control above.  */

	}

      while (__glibc_unlikely (!atomic_compare_exchange_weak_acquire (

	  once_control, &val, newval)));

      /* Check if another thread already runs the initializer.	*/

      if ((val & __PTHREAD_ONCE_INPROGRESS) != 0)

	{

	  /* Check whether the initializer execution was interrupted by a

	     fork.  We know that for both values, __PTHREAD_ONCE_INPROGRESS

	     is set and __PTHREAD_ONCE_DONE is not.  */

	  if (val == newval)

	    {

	      /* Same generation, some other thread was faster.  Wait and

		 retry.  */

	      futex_wait_simple ((unsigned int *) once_control,

				 (unsigned int) newval, FUTEX_PRIVATE);

	      continue;

	    }

	}

      /* This thread is the first here.  Do the initialization.

	 Register a cleanup handler so that in case the thread gets

	 interrupted the initialization can be restarted.  */

      pthread_cleanup_push (clear_once_control, once_control);

      init_routine ();

      pthread_cleanup_pop (0);

      /* Mark *once_control as having finished the initialization.  We need

         release memory order here because we need to synchronize with other

         threads that want to use the initialized data.  */

      atomic_store_release (once_control, __PTHREAD_ONCE_DONE);

      /* Wake up all other threads.  */

      futex_wake ((unsigned int *) once_control, INT_MAX, FUTEX_PRIVATE);

      break;

    }

  return 0;

}

int

__pthread_once (pthread_once_t *once_control, void (*init_routine) (void))

{

  /* Fast path.  See __pthread_once_slow.  */

  int val;

  val = atomic_load_acquire (once_control);

  if (__glibc_likely ((val & __PTHREAD_ONCE_DONE) != 0))

    return 0;

  else

    return __pthread_once_slow (once_control, init_routine);

}

weak_alias (__pthread_once, pthread_once)

hidden_def (__pthread_once)