From ccad6013892a95574e7c3ec652f3e03c357d10b7 Mon Sep 17 00:00:00 2001
From: Marco Costalba <mcostalba@gmail.com>
Date: Mon, 4 Mar 2013 08:58:57 +0100
Subject: [PATCH] Avoid locking/unlocking in a tight loop

After previous patch if split point master is
waiting for job and "Use Sleeping Threads" is
false (our condition for official releases) then
it will lock/unlock splitPoint mutex in a super
tight loop badly affecting performance.

Rewrite the code to lock only when we are about
to finish. Note that race condition on slavesMask
is anyhow fixed.

No functional change.
---
 src/search.cpp | 17 ++++++++++-------
 src/thread.cpp |  5 -----
 2 files changed, 10 insertions(+), 12 deletions(-)

diff --git a/src/search.cpp b/src/search.cpp
index c5c5b944..9a373dec 100644
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -1626,13 +1626,8 @@ void Thread::idle_loop() {
 
   assert(!this_sp || (this_sp->masterThread == this && searching));
 
-  // If this thread is the master of a split point and all slaves have finished
-  // their work at this split point, return from the idle loop.
-  while (!this_sp || this_sp->slavesMask)
+  while (true)
   {
-      if (this_sp)
-          this_sp->mutex.unlock();
-
       // If we are not searching, wait for a condition to be signaled instead of
       // wasting CPU time polling for work.
       while ((!searching && Threads.sleepWhileIdle) || exit)
@@ -1725,8 +1720,16 @@ void Thread::idle_loop() {
           sp->mutex.unlock();
       }
 
-      if(this_sp)
+      // If this thread is the master of a split point and all slaves have finished
+      // their work at this split point, return from the idle loop.
+      if (this_sp && !this_sp->slavesMask)
+      {
           this_sp->mutex.lock();
+          bool finished = !this_sp->slavesMask; // Retest under lock protection
+          this_sp->mutex.unlock();
+          if (finished)
+              return;
+      }
   }
 }
 
diff --git a/src/thread.cpp b/src/thread.cpp
index df746a4a..0d8070f2 100644
--- a/src/thread.cpp
+++ b/src/thread.cpp
@@ -312,7 +312,6 @@ void Thread::split(Position& pos, Stack* ss, Value alpha, Value beta, Value* bes
       sp.mutex.unlock();
       Threads.mutex.unlock();
 
-      // Calling idle_loop with sp.mutex locked
       Thread::idle_loop(); // Force a call to base class idle_loop()
 
       // In helpful master concept a master can help only a sub-tree of its split
@@ -323,10 +322,6 @@ void Thread::split(Position& pos, Stack* ss, Value alpha, Value beta, Value* bes
       // We have returned from the idle loop, which means that all threads are
       // finished. Note that setting 'searching' and decreasing splitPointsSize is
       // done under lock protection to avoid a race with Thread::is_available_to().
-      // idle_loop returns with sp.mutex locked but we must unlock it inorder to
-      // lock Threads.mutex without conflicting with check_time() (threads holding 
-      // multiple locks must always acquired them in the same order to avoid deadlocks)
-      sp.mutex.unlock(); 
       Threads.mutex.lock();
       sp.mutex.lock();
   }
-- 
2.39.2