[Glibc-bsd-commits] r5490 - trunk/glibc-ports/kfreebsd/fbtl

[ps-guest at alioth.debian.org]

Author: ps-guest
Date: 2014-07-12 08:37:53 +0000 (Sat, 12 Jul 2014)
New Revision: 5490

Modified:
   trunk/glibc-ports/kfreebsd/fbtl/pthread_once.c
Log:
sync pthread_once.c with current generic nptl version


Modified: trunk/glibc-ports/kfreebsd/fbtl/pthread_once.c
===================================================================
--- trunk/glibc-ports/kfreebsd/fbtl/pthread_once.c	2014-07-12 08:37:06 UTC (rev 5489)
+++ trunk/glibc-ports/kfreebsd/fbtl/pthread_once.c	2014-07-12 08:37:53 UTC (rev 5490)
@@ -1,4 +1,4 @@
-/* Copyright (C) 2003-2013 Free Software Foundation, Inc.
+/* Copyright (C) 2003-2014 Free Software Foundation, Inc.
    This file is part of the GNU C Library.
    Contributed by Jakub Jelinek <jakub at redhat.com>, 2003.
 
@@ -18,6 +18,7 @@
 
 #include "pthreadP.h"
 #include <lowlevellock.h>
+#include <atomic.h>
 
 
 unsigned long int __fork_generation attribute_hidden;
@@ -28,11 +29,33 @@
 {
   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;
@@ -42,26 +65,38 @@
     {
       int oldval, val, newval;
 
+      /* 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
 	{
-	  /* Check if the initialized has already been done.  */
-	  if ((val & 2) != 0)
+	  /* Check if the initialization has already been done.  */
+	  if (__glibc_likely ((val & 2) != 0))
 	    return 0;
 
 	  oldval = val;
-	  newval = (oldval & 3) | __fork_generation | 1;
+	  /* 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 (__builtin_expect (val != oldval, 0));
+      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.	 */
-	  if (((oldval ^ newval) & -4) == 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);
@@ -79,8 +114,11 @@
       pthread_cleanup_pop (0);
 
 
-      /* Add one to *once_control.  */
-      atomic_increment (once_control);
+      /* 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);