Get rid of nptl/sysdeps/ entirely!

nptl/pthread_once.c

@@ -1,6 +1,6 @@
-/* Copyright (C) 2002-2014 Free Software Foundation, Inc.
+/* Copyright (C) 2003-2014 Free Software Foundation, Inc.
    This file is part of the GNU C Library.
-   Contributed by Ulrich Drepper <drepper@redhat.com>, 2002.
+   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
@@ -9,7 +9,7 @@
 
    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
+   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
@@ -18,37 +18,114 @@
 
 #include "pthreadP.h"
 #include <lowlevellock.h>
+#include <atomic.h>
 
 
-
-static int once_lock = LLL_LOCK_INITIALIZER;
+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.  */
+  *once_control = 0;
+  lll_futex_wake (once_control, INT_MAX, LLL_PRIVATE);
+}
+
+
+/* This is similar to a lock implementation, but we distinguish between three
+   states: not yet initialized (0), initialization finished (2), and
+   initialization in progress (__fork_generation | 1).  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.  */
 int
 __pthread_once (once_control, init_routine)
      pthread_once_t *once_control;
      void (*init_routine) (void);
 {
-  /* XXX Depending on whether the LOCK_IN_ONCE_T is defined use a
-     global lock variable or one which is part of the pthread_once_t
-     object.  */
-  if (*once_control == PTHREAD_ONCE_INIT)
+  while (1)
     {
-      lll_lock (once_lock, LLL_PRIVATE);
+      int oldval, val, newval;
 
-      /* XXX This implementation is not complete.  It doesn't take
-	 cancelation and fork into account.  */
-      if (*once_control == PTHREAD_ONCE_INIT)
+      /* We need acquire memory order for this load because if the value
+         signals that initialization has finished, we need to be see any
+         data modifications done during initialization.  */
+      val = *once_control;
+      atomic_read_barrier();
+      do
 	{
-	  init_routine ();
+	  /* Check if the initialization has already been done.  */
+	  if (__glibc_likely ((val & 2) != 0))
+	    return 0;
 
-	  *once_control = !PTHREAD_ONCE_INIT;
+	  oldval = val;
+	  /* 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 | 1;
+	  /* We need acquire memory order here for the same reason as for the
+	     load from once_control above.  */
+	  val = atomic_compare_and_exchange_val_acq (once_control, newval,
+						     oldval);
+	}
+      while (__glibc_unlikely (val != oldval));
+
+      /* Check if another thread already runs the initializer.	*/
+      if ((oldval & 1) != 0)
+	{
+	  /* Check whether the initializer execution was interrupted by a
+	     fork.  We know that for both values, bit 0 is set and bit 1 is
+	     not.  */
+	  if (oldval == newval)
+	    {
+	      /* Same generation, some other thread was faster. Wait.  */
+	      lll_futex_wait (once_control, newval, LLL_PRIVATE);
+	      continue;
+	    }
 	}
 
-      lll_unlock (once_lock, LLL_PRIVATE);
+      /* 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_write_barrier();
+      *once_control = 2;
+
+      /* Wake up all other threads.  */
+      lll_futex_wake (once_control, INT_MAX, LLL_PRIVATE);
+      break;
     }
 
   return 0;
 }
-strong_alias (__pthread_once, pthread_once)
+weak_alias (__pthread_once, pthread_once)
 hidden_def (__pthread_once)