S390: Adjust lock elision code after review.

This patch adjusts s390 specific lock elision code after review
of the following patches:
-S390: Use own tbegin macro instead of __builtin_tbegin.
(8bfc4a2ab4bebdf86c151665aae8a266e2f18fb4)
-S390: Use new __libc_tbegin_retry macro in elision-lock.c.
(53c5c3d5ac238901c13f28a73ba05b0678094e80)
-S390: Optimize lock-elision by decrementing adapt_count at unlock.
(dd037fb3df286b7c2d0b0c6f8d02a2dd8a8e8a08)

The futex value is not tested before starting a transaction,
__glibc_likely is used instead of __builtin_expect and comments
are adjusted.

ChangeLog:

	* sysdeps/unix/sysv/linux/s390/htm.h: Adjust comments.
	* sysdeps/unix/sysv/linux/s390/elision-unlock.c: Likewise.
	* sysdeps/unix/sysv/linux/s390/elision-lock.c: Adjust comments.
	(__lll_lock_elision): Do not test futex before starting a
	transaction.  Use __glibc_likely instead of __builtin_expect.
	* sysdeps/unix/sysv/linux/s390/elision-trylock.c: Adjust comments.
	(__lll_trylock_elision): Do not test futex before starting a
	transaction.  Use __glibc_likely instead of __builtin_expect.

1 files changed, 14 insertions, 10 deletions

diff --git a/sysdeps/unix/sysv/linux/s390/elision-unlock.c b/sysdeps/unix/sysv/linux/s390/elision-unlock.c
index d9205faf52..c062d71b77 100644
--- a/sysdeps/unix/sysv/linux/s390/elision-unlock.c
+++ b/sysdeps/unix/sysv/linux/s390/elision-unlock.c
@@ -26,8 +26,12 @@ __lll_unlock_elision(int *futex, short *adapt_count, int private)
   /* If the lock is free, we elided the lock earlier.  This does not
      necessarily mean that we are in a transaction, because the user code may
      have closed the transaction, but that is impossible to detect reliably.
-     Relaxed MO access to futex is sufficient as we only need a hint, if we
-     started a transaction or acquired the futex in e.g. elision-lock.c.  */
+     Relaxed MO access to futex is sufficient because a correct program
+     will only release a lock it has acquired; therefore, it must either
+     changed the futex word's value to something !=0 or it must have used
+     elision; these are actions by the same thread, so these actions are
+     sequenced-before the relaxed load (and thus also happens-before the
+     relaxed load).  Therefore, relaxed MO is sufficient.  */
   if (atomic_load_relaxed (futex) == 0)
     {
       __libc_tend ();
@@ -36,17 +40,17 @@ __lll_unlock_elision(int *futex, short *adapt_count, int private)
     {
       /* Update the adapt_count while unlocking before completing the critical
 	 section.  adapt_count is accessed concurrently outside of a
-	 transaction or an aquired lock e.g. in elision-lock.c so we need to use
-	 atomic accesses.  However, the value of adapt_count is just a hint, so
-	 relaxed MO accesses are sufficient.
+	 transaction or a critical section (e.g. in elision-lock.c). So we need
+	 to use atomic accesses.  However, the value of adapt_count is just a
+	 hint, so relaxed MO accesses are sufficient.
 	 If adapt_count would be decremented while locking, multiple
-	 CPUs trying to lock the locked mutex will decrement adapt_count to
+	 CPUs, trying to lock the acquired mutex, will decrement adapt_count to
 	 zero and another CPU will try to start a transaction, which will be
 	 immediately aborted as the mutex is locked.
-	 If adapt_count would be decremented while unlocking after completing
-	 the critical section, possible waiters will be waked up before
-	 decrementing the adapt_count.  Those waked up waiters could have
-	 destroyed and freed this mutex!  */
+	 The update of adapt_count is done before releasing the lock as POSIX'
+	 mutex destruction requirements disallow accesses to the mutex after it
+	 has been released and thus could have been acquired or destroyed by
+	 another thread.  */
       short adapt_count_val = atomic_load_relaxed (adapt_count);
       if (adapt_count_val > 0)
 	atomic_store_relaxed (adapt_count, adapt_count_val - 1);