PowerPC: Fix a race condition when eliding a lock

The previous code used to evaluate the preprocessor token is_lock_free to
a variable before starting a transaction.  This behavior can cause an
error if another thread got the lock (without using a transaction)
between the evaluation of the token and the beginning of the transaction.

This bug can be triggered with the following order of events:
1. The lock accessed by is_lock_free is free.
2. Thread T1 evaluates is_lock_free and stores into register R1 that the
   lock is free.
3. Thread T2 acquires the same lock used in is_lock_free.
4. T1 begins the transaction, creating a memory barrier where is_lock_free
   is false, but R1 is true.
5. T1 reads R1 and doesn't abort the transaction.
6. T1 calls ELIDE_UNLOCK, which reads false from is_lock_free and decides
   to unlock a lock acquired by T2, leading to undefined behavior.

This patch delays the evaluation of is_lock_free to inside a transaction
by moving this part of the code to the macro ELIDE_LOCK.

	[BZ #18743]
	* sysdeps/powerpc/nptl/elide.h (__elide_lock): Move most of this
	code to...
	(ELIDE_LOCK): ...here.
	(__get_new_count): New function with part of the code from
	__elide_lock that updates the value of adapt_count after a
	transaction abort.
	(__elided_trylock): Moved this code to...
	(ELIDE_TRYLOCK): ...here.

ChangeLog

@@ -1,3 +1,15 @@
+2015-10-19  Tulio Magno Quites Machado Filho  <tuliom@linux.vnet.ibm.com>
+
+	[BZ #18743]
+	* sysdeps/powerpc/nptl/elide.h (__elide_lock): Move most of this
+	code to...
+	(ELIDE_LOCK): ...here.
+	(__get_new_count): New function with part of the code from
+	__elide_lock that updates the value of adapt_count after a
+	transaction abort.
+	(__elided_trylock): Moved this code to...
+	(ELIDE_TRYLOCK): ...here.
+
 2015-10-19  Mike Frysinger  <vapier@gentoo.org>
 
 	* configure.ac (AC_ARG_ENABLE(timezone-tools)): Tweak help phrasing.

NEWS

@@ -14,13 +14,13 @@ Version 2.23
   16517, 16519, 16520, 16521, 16620, 16734, 16973, 16985, 17118, 17243,
   17244, 17250, 17441, 17787, 17886, 17887, 17905, 18084, 18086, 18240,
   18265, 18370, 18421, 18480, 18525, 18595, 18589, 18610, 18618, 18647,
-  18661, 18674, 18675, 18681, 18724, 18757, 18778, 18781, 18787, 18789,
-  18790, 18795, 18796, 18803, 18820, 18823, 18824, 18825, 18857, 18863,
-  18870, 18872, 18873, 18875, 18887, 18918, 18921, 18928, 18951, 18952,
-  18953, 18956, 18961, 18966, 18967, 18969, 18970, 18977, 18980, 18981,
-  18982, 18985, 19003, 19007, 19012, 19016, 19018, 19032, 19046, 19049,
-  19050, 19059, 19071, 19074, 19076, 19077, 19078, 19079, 19085, 19086,
-  19088, 19094, 19095, 19124, 19125, 19129, 19134, 19137.
+  18661, 18674, 18675, 18681, 18724, 18743, 18757, 18778, 18781, 18787,
+  18789, 18790, 18795, 18796, 18803, 18820, 18823, 18824, 18825, 18857,
+  18863, 18870, 18872, 18873, 18875, 18887, 18918, 18921, 18928, 18951,
+  18952, 18953, 18956, 18961, 18966, 18967, 18969, 18970, 18977, 18980,
+  18981, 18982, 18985, 19003, 19007, 19012, 19016, 19018, 19032, 19046,
+  19049, 19050, 19059, 19071, 19074, 19076, 19077, 19078, 19079, 19085,
+  19086, 19088, 19094, 19095, 19124, 19125, 19129, 19134, 19137.
 
 * There is now a --disable-timezone-tools configure option for disabling the
   building and installing of the timezone related utilities (zic, zdump, and

sysdeps/powerpc/nptl/elide.h

@@ -23,67 +23,78 @@
 # include <htm.h>
 # include <elision-conf.h>
 
-/* Returns true if the lock defined by is_lock_free as elided.
-   ADAPT_COUNT is a pointer to per-lock state variable. */
-
+/* Get the new value of adapt_count according to the elision
+   configurations.  Returns true if the system should retry again or false
+   otherwise.  */
 static inline bool
-__elide_lock (uint8_t *adapt_count, int is_lock_free)
+__get_new_count (uint8_t *adapt_count)
 {
-  if (*adapt_count > 0)
+  /* A persistent failure indicates that a retry will probably
+     result in another failure.  Use normal locking now and
+     for the next couple of calls.  */
+  if (_TEXASRU_FAILURE_PERSISTENT (__builtin_get_texasru ()))
     {
-      (*adapt_count)--;
+      if (__elision_aconf.skip_lock_internal_abort > 0)
+	*adapt_count = __elision_aconf.skip_lock_internal_abort;
       return false;
     }
-
-  for (int i = __elision_aconf.try_tbegin; i > 0; i--)
-    {
-      if (__builtin_tbegin (0))
-	{
-	  if (is_lock_free)
-	    return true;
-	  /* Lock was busy.  */
-	  __builtin_tabort (_ABORT_LOCK_BUSY);
-	}
-      else
-	{
-	  /* A persistent failure indicates that a retry will probably
-	     result in another failure.  Use normal locking now and
-	     for the next couple of calls.  */
-	  if (_TEXASRU_FAILURE_PERSISTENT (__builtin_get_texasru ()))
-	    {
-	      if (__elision_aconf.skip_lock_internal_abort > 0)
-		*adapt_count = __elision_aconf.skip_lock_internal_abort;
-	      break;
-	    }
-	  /* Same logic as above, but for a number of temporary failures in a
-	     a row.  */
-	  else if (__elision_aconf.skip_lock_out_of_tbegin_retries > 0
-		   && __elision_aconf.try_tbegin > 0)
-	    *adapt_count = __elision_aconf.skip_lock_out_of_tbegin_retries;
-	}
-     }
-
-  return false;
+  /* Same logic as above, but for a number of temporary failures in a
+     a row.  */
+  else if (__elision_aconf.skip_lock_out_of_tbegin_retries > 0
+	   && __elision_aconf.try_tbegin > 0)
+    *adapt_count = __elision_aconf.skip_lock_out_of_tbegin_retries;
+  return true;
 }
 
-# define ELIDE_LOCK(adapt_count, is_lock_free) \
-  __elide_lock (&(adapt_count), is_lock_free)
-
-
-static inline bool
-__elide_trylock (uint8_t *adapt_count, int is_lock_free, int write)
-{
-  if (__elision_aconf.try_tbegin > 0)
-    {
-      if (write)
-	__builtin_tabort (_ABORT_NESTED_TRYLOCK);
-      return __elide_lock (adapt_count, is_lock_free);
-    }
-  return false;
-}
+/* CONCURRENCY NOTES:
+
+   The evaluation of the macro expression is_lock_free encompasses one or
+   more loads from memory locations that are concurrently modified by other
+   threads.  For lock elision to work, this evaluation and the rest of the
+   critical section protected by the lock must be atomic because an
+   execution with lock elision must be equivalent to an execution in which
+   the lock would have been actually acquired and released.  Therefore, we
+   evaluate is_lock_free inside of the transaction that represents the
+   critical section for which we want to use lock elision, which ensures
+   the atomicity that we require.  */
+
+/* Returns 0 if the lock defined by is_lock_free was elided.
+   ADAPT_COUNT is a per-lock state variable.  */
+# define ELIDE_LOCK(adapt_count, is_lock_free)				\
+  ({									\
+    int ret = 0;							\
+    if (adapt_count > 0)						\
+      (adapt_count)--;							\
+    else								\
+      for (int i = __elision_aconf.try_tbegin; i > 0; i--)		\
+	{								\
+	  if (__builtin_tbegin (0))					\
+	    {								\
+	      if (is_lock_free)						\
+		{							\
+		  ret = 1;						\
+		  break;						\
+		}							\
+	      __builtin_tabort (_ABORT_LOCK_BUSY);			\
+	    }								\
+	  else								\
+	    if (!__get_new_count(&adapt_count))				\
+	      break;							\
+	}								\
+    ret;								\
+  })
 
 # define ELIDE_TRYLOCK(adapt_count, is_lock_free, write)	\
-  __elide_trylock (&(adapt_count), is_lock_free, write)
+  ({								\
+    int ret = 0;						\
+    if (__elision_aconf.try_tbegin > 0)				\
+      {								\
+	if (write)						\
+	  __builtin_tabort (_ABORT_NESTED_TRYLOCK);		\
+	ret = ELIDE_LOCK (adapt_count, is_lock_free);		\
+      }								\
+    ret;							\
+  })
 
 
 static inline bool