Add the low level infrastructure for pthreads lock elision with TSX

Lock elision using TSX is a technique to optimize lock scaling
It allows to run locks in parallel using hardware support for
a transactional execution mode in 4th generation Intel Core CPUs.
See http://www.intel.com/software/tsx for more Information.

This patch implements a simple adaptive lock elision algorithm based
on RTM. It enables elision for the pthread mutexes and rwlocks.
The algorithm keeps track whether a mutex successfully elides or not,
and stops eliding for some time when it is not.

When the CPU supports RTM the elision path is automatically tried,
otherwise any elision is disabled.

The adaptation algorithm and its tuning is currently preliminary.

The code adds some checks to the lock fast paths. Micro-benchmarks
show little to no difference without RTM.

This patch implements the low level "lll_" code for lock elision.
Followon patches hook this into the pthread implementation

Changes with the RTM mutexes:
-----------------------------
Lock elision in pthreads is generally compatible with existing programs.
There are some obscure exceptions, which are expected to be uncommon.
See the manual for more details.

- A broken program that unlocks a free lock will crash.
  There are ways around this with some tradeoffs (more code in hot paths)
  I'm still undecided on what approach to take here; have to wait for testing reports.
- pthread_mutex_destroy of a lock mutex will not return EBUSY but 0.
- There's also a similar situation with trylock outside the mutex,
  "knowing" that the mutex must be held due to some other condition.
  In this case an assert failure cannot be recovered. This situation is
  usually an existing bug in the program.
- Same applies to the rwlocks. Some of the return values changes
  (for example there is no EDEADLK for an elided lock, unless it aborts.
   However when elided it will also never deadlock of course)
- Timing changes, so broken programs that make assumptions about specific timing
  may expose already existing latent problems.  Note that these broken programs will
  break in other situations too (loaded system, new faster hardware, compiler
  optimizations etc.)
- Programs with non recursive mutexes that take them recursively in a thread and
  which would always deadlock without elision may not always see a deadlock.
  The deadlock will only happen on an early or delayed abort (which typically
  happens at some point)
  This only happens for mutexes not explicitely set to PTHREAD_MUTEX_NORMAL
  or PTHREAD_MUTEX_ADAPTIVE_NP.  PTHREAD_MUTEX_NORMAL mutexes do not elide.

The elision default can be set at configure time.

This patch implements the basic infrastructure for elision.

1 files changed, 95 insertions, 0 deletions

diff --git a/nptl/sysdeps/unix/sysv/linux/x86/elision-lock.c b/nptl/sysdeps/unix/sysv/linux/x86/elision-lock.c
new file mode 100644
index 0000000000..de16f1b02f
--- /dev/null
+++ b/nptl/sysdeps/unix/sysv/linux/x86/elision-lock.c
@@ -0,0 +1,95 @@
+/* elision-lock.c: Elided pthread mutex lock.
+   Copyright (C) 2011-2013 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>. */
+
+#include <pthread.h>
+#include "pthreadP.h"
+#include "lowlevellock.h"
+#include "hle.h"
+#include <elision-conf.h>
+
+#if !defined(LLL_LOCK) && !defined(EXTRAARG)
+/* Make sure the configuration code is always linked in for static
+   libraries.  */
+#include "elision-conf.c"
+#endif
+
+#ifndef EXTRAARG
+#define EXTRAARG
+#endif
+#ifndef LLL_LOCK
+#define LLL_LOCK(a,b) lll_lock(a,b), 0
+#endif
+
+#define aconf __elision_aconf
+
+/* Adaptive lock using transactions.
+   By default the lock region is run as a transaction, and when it
+   aborts or the lock is busy the lock adapts itself.  */
+
+int
+__lll_lock_elision (int *futex, short *adapt_count, EXTRAARG int private)
+{
+  if (*adapt_count <= 0)
+    {
+      unsigned status;
+      int try_xbegin;
+
+      for (try_xbegin = aconf.retry_try_xbegin;
+	   try_xbegin > 0;
+	   try_xbegin--)
+	{
+	  if ((status = _xbegin()) == _XBEGIN_STARTED)
+	    {
+	      if (*futex == 0)
+		return 0;
+
+	      /* Lock was busy. Fall back to normal locking.
+		 Could also _xend here but xabort with 0xff code
+		 is more visible in the profiler.  */
+	      _xabort (_ABORT_LOCK_BUSY);
+	    }
+
+	  if (!(status & _XABORT_RETRY))
+	    {
+	      if ((status & _XABORT_EXPLICIT)
+			&& _XABORT_CODE (status) == _ABORT_LOCK_BUSY)
+	        {
+		  /* Right now we skip here. Better would be to wait a bit
+		     and retry. This likely needs some spinning.  */
+		  if (*adapt_count != aconf.skip_lock_busy)
+		    *adapt_count = aconf.skip_lock_busy;
+		}
+	      /* Internal abort. There is no chance for retry.
+		 Use the normal locking and next time use lock.
+		 Be careful to avoid writing to the lock.  */
+	      else if (*adapt_count != aconf.skip_lock_internal_abort)
+		*adapt_count = aconf.skip_lock_internal_abort;
+	      break;
+	    }
+	}
+    }
+  else
+    {
+      /* Use a normal lock until the threshold counter runs out.
+	 Lost updates possible.  */
+      (*adapt_count)--;
+    }
+
+  /* Use a normal lock here.  */
+  return LLL_LOCK ((*futex), private);
+}