commit abdf7e3d264e56898f474405602205279895d9b6
Author: Stefan Liebler <stli@linux.vnet.ibm.com>
Date:   Thu Jan 19 10:44:18 2017 +0100

    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.

diff --git a/sysdeps/unix/sysv/linux/s390/elision-lock.c b/sysdeps/unix/sysv/linux/s390/elision-lock.c
index a7c0fcc..0081537 100644
--- a/sysdeps/unix/sysv/linux/s390/elision-lock.c
+++ b/sysdeps/unix/sysv/linux/s390/elision-lock.c
@@ -50,27 +50,28 @@ __lll_lock_elision (int *futex, short *adapt_count, EXTRAARG int private)
      critical section uses lock elision) and outside of transactions.  Thus,
      we need to use atomic accesses to avoid data races.  However, the
      value of adapt_count is just a hint, so relaxed MO accesses are
-     sufficient.
-     Do not begin a transaction if another cpu has locked the
-     futex with normal locking.  If adapt_count is zero, it remains and the
-     next pthread_mutex_lock call will try to start a transaction again.  */
-    if (atomic_load_relaxed (futex) == 0
-	&& atomic_load_relaxed (adapt_count) <= 0 && aconf.try_tbegin > 0)
+     sufficient.  */
+    if (atomic_load_relaxed (adapt_count) <= 0 && aconf.try_tbegin > 0)
     {
+      /* Start a transaction and retry it automatically if it aborts with
+	 _HTM_TBEGIN_TRANSIENT.  This macro calls tbegin at most retry_cnt
+	 + 1 times.  The second argument is considered as retry_cnt.  */
       int status = __libc_tbegin_retry ((void *) 0, aconf.try_tbegin - 1);
-      if (__builtin_expect (status == _HTM_TBEGIN_STARTED,
-			    _HTM_TBEGIN_STARTED))
+      if (__glibc_likely (status == _HTM_TBEGIN_STARTED))
 	{
 	  /* Check the futex to make sure nobody has touched it in the
 	     mean time.  This forces the futex into the cache and makes
-	     sure the transaction aborts if some other cpu uses the
-	     lock (writes the futex).  */
-	  if (__builtin_expect (atomic_load_relaxed (futex) == 0, 1))
+	     sure the transaction aborts if another thread acquires the lock
+	     concurrently.  */
+	  if (__glibc_likely (atomic_load_relaxed (futex) == 0))
 	    /* Lock was free.  Return to user code in a transaction.  */
 	    return 0;
 
-	  /* Lock was busy.  Fall back to normal locking.  */
-	  if (__builtin_expect (__libc_tx_nesting_depth () <= 1, 1))
+	  /* Lock was busy.  Fall back to normal locking.
+	     This can be the case if e.g. adapt_count was decremented to zero
+	     by a former release and another thread has been waken up and
+	     acquired it.  */
+	  if (__glibc_likely (__libc_tx_nesting_depth () <= 1))
 	    {
 	      /* In a non-nested transaction there is no need to abort,
 		 which is expensive.  Simply end the started transaction.  */
@@ -118,6 +119,7 @@ __lll_lock_elision (int *futex, short *adapt_count, EXTRAARG int private)
 	}
     }
 
-  /* Use normal locking as fallback path if transaction does not succeed.  */
+  /* Use normal locking as fallback path if the transaction does not
+     succeed.  */
   return LLL_LOCK ((*futex), private);
 }
diff --git a/sysdeps/unix/sysv/linux/s390/elision-trylock.c b/sysdeps/unix/sysv/linux/s390/elision-trylock.c
index 3c1d009..aa09073 100644
--- a/sysdeps/unix/sysv/linux/s390/elision-trylock.c
+++ b/sysdeps/unix/sysv/linux/s390/elision-trylock.c
@@ -51,31 +51,29 @@ __lll_trylock_elision (int *futex, short *adapt_count)
      critical section uses lock elision) and outside of transactions.  Thus,
      we need to use atomic accesses to avoid data races.  However, the
      value of adapt_count is just a hint, so relaxed MO accesses are
-     sufficient.
-     Do not begin a transaction if another cpu has locked the
-     futex with normal locking.  If adapt_count is zero, it remains and the
-     next pthread_mutex_lock call will try to start a transaction again.  */
-    if (atomic_load_relaxed (futex) == 0
-	&& atomic_load_relaxed (adapt_count) <= 0 && aconf.try_tbegin > 0)
+     sufficient.  */
+    if (atomic_load_relaxed (adapt_count) <= 0 && aconf.try_tbegin > 0)
     {
       int status = __libc_tbegin ((void *) 0);
-      if (__builtin_expect (status  == _HTM_TBEGIN_STARTED,
-			    _HTM_TBEGIN_STARTED))
+      if (__glibc_likely (status  == _HTM_TBEGIN_STARTED))
 	{
 	  /* Check the futex to make sure nobody has touched it in the
 	     mean time.  This forces the futex into the cache and makes
-	     sure the transaction aborts if some other cpu uses the
-	     lock (writes the futex).  */
-	  if (__builtin_expect (atomic_load_relaxed (futex) == 0, 1))
+	     sure the transaction aborts if another thread acquires the lock
+	     concurrently.  */
+	  if (__glibc_likely (atomic_load_relaxed (futex) == 0))
 	    /* Lock was free.  Return to user code in a transaction.  */
 	    return 0;
 
-	  /* Lock was busy.  Fall back to normal locking.  Since we are in
-	     a non-nested transaction there is no need to abort, which is
-	     expensive.  Simply end the started transaction.  */
+	  /* Lock was busy.  Fall back to normal locking.
+	     This can be the case if e.g. adapt_count was decremented to zero
+	     by a former release and another thread has been waken up and
+	     acquired it.
+	     Since we are in a non-nested transaction there is no need to abort,
+	     which is expensive.  Simply end the started transaction.  */
 	  __libc_tend ();
 	  /* Note: Changing the adapt_count here might abort a transaction on a
-	     different cpu, but that could happen anyway when the futex is
+	     different CPU, but that could happen anyway when the futex is
 	     acquired, so there's no need to check the nesting depth here.
 	     See above for why relaxed MO is sufficient.  */
 	  if (aconf.skip_lock_busy > 0)
@@ -93,6 +91,7 @@ __lll_trylock_elision (int *futex, short *adapt_count)
       /* Could do some retries here.  */
     }
 
-  /* Use normal locking as fallback path if transaction does not succeed.  */
+  /* Use normal locking as fallback path if the transaction does not
+     succeed.  */
   return lll_trylock (*futex);
 }
diff --git a/sysdeps/unix/sysv/linux/s390/elision-unlock.c b/sysdeps/unix/sysv/linux/s390/elision-unlock.c
index d9205fa..c062d71 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);
diff --git a/sysdeps/unix/sysv/linux/s390/htm.h b/sysdeps/unix/sysv/linux/s390/htm.h
index 32d5a88..70d7f66 100644
--- a/sysdeps/unix/sysv/linux/s390/htm.h
+++ b/sysdeps/unix/sysv/linux/s390/htm.h
@@ -119,11 +119,16 @@
 			      ar modification and fp operations.  Some	\
 			      program-interruptions (e.g. a null	\
 			      pointer access) are filtered and the	\
-			      trancsaction will abort.  In this case	\
+			      transaction will abort.  In this case	\
 			      the normal lock path will execute it	\
 			      again and result in a core dump wich does	\
 			      now show at tbegin but the real executed	\
-			      instruction.  */				\
+			      instruction.				\
+			      However it is not guaranteed that this	\
+			      retry operate on the same data and thus	\
+			      may not end in an program-interruption.	\
+			      Note: This could also be used to probe	\
+			      memory for being accessible!  */		\
 			   "2: tbegin 0, 0xFF0E\n\t"			\
 			   /* Branch away in abort case (this is the	\
 			      prefered sequence.  See PoP in chapter 5	\
@@ -152,7 +157,8 @@
      __ret;								\
      })
 
-/* These builtins are correct.  Use them.  */
+/* These builtins are usable in context of glibc lock elision code without any
+   changes.  Use them.  */
 #define __libc_tend()							\
   ({ __asm__ __volatile__ (".machine push\n\t"				\
 			   ".machinemode \"zarch_nohighgprs\"\n\t"	\
