Prepare for radical source tree reorganization. zack/build-layout-experiment

All top-level files and directories are moved into a temporary storage
directory, REORG.TODO, except for files that will certainly still
exist in their current form at top level when we're done (COPYING,
COPYING.LIB, LICENSES, NEWS, README), all old ChangeLog files (which
are moved to the new directory OldChangeLogs, instead), and the
generated file INSTALL (which is just deleted; in the new order, there
will be no generated files checked into version control).

1 files changed, 146 insertions, 0 deletions

diff --git a/REORG.TODO/nptl/pthread_once.c b/REORG.TODO/nptl/pthread_once.c
new file mode 100644
index 0000000000..fbc51ca0b2
--- /dev/null
+++ b/REORG.TODO/nptl/pthread_once.c
@@ -0,0 +1,146 @@
+/* Copyright (C) 2003-2017 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)