Document a behavior of an elided pthread_rwlock_unlock

Explain that pthread_rwlock_unlock may crash if called on a lock not
held by the current thread.

3 files changed, 13 insertions, 0 deletions

diff --git a/ChangeLog b/ChangeLog
index 4f76008..d5b3f7b 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,11 @@
 2016-10-28  Tulio Magno Quites Machado Filho  <tuliom@linux.vnet.ibm.com>
 
+	* nptl/pthread_rwlock_unlock.c: Add a comment explaining its
+	behavior when eliding a lock not held by the current thread.
+	* sysdeps/powerpc/nptl/elide.h: Likewise.
+
+2016-10-28  Tulio Magno Quites Machado Filho  <tuliom@linux.vnet.ibm.com>
+
 	[BZ #20728]
 	* sysdeps/unix/sysv/linux/powerpc/powerpc64/clone.S: Replace a
 	branch to _exit() by a function call.
diff --git a/nptl/pthread_rwlock_unlock.c b/nptl/pthread_rwlock_unlock.c
index a6cadd4..112f748 100644
--- a/nptl/pthread_rwlock_unlock.c
+++ b/nptl/pthread_rwlock_unlock.c
@@ -35,6 +35,10 @@ __pthread_rwlock_unlock (pthread_rwlock_t *rwlock)
 
   LIBC_PROBE (rwlock_unlock, 1, rwlock);
 
+  /* Trying to elide an unlocked lock may crash the process.  This
+     is expected and is compatible with POSIX.1-2008: "results are
+     undefined if the read-write lock rwlock is not held by the
+     calling thread".  */
   if (ELIDE_UNLOCK (rwlock->__data.__writer == 0
 		    && rwlock->__data.__nr_readers == 0))
     return 0;
diff --git a/sysdeps/powerpc/nptl/elide.h b/sysdeps/powerpc/nptl/elide.h
index 77bd82e..835fd92 100644
--- a/sysdeps/powerpc/nptl/elide.h
+++ b/sysdeps/powerpc/nptl/elide.h
@@ -102,6 +102,9 @@ __elide_unlock (int is_lock_free)
 {
   if (is_lock_free)
     {
+      /* This code is expected to crash when trying to unlock a lock not
+	 held by this thread.  More information is available in the
+	 __pthread_rwlock_unlock() implementation.  */
       __libc_tend (0);
       return true;
     }