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Fix 'stop' flag changed out of lock protection

This is the first nice effect of previous patch !

Because thread_should_stop() should be declared 'const' we
need to remove the setting of 'stop' flag to true that
turns out to be a bug because thread_should_stop() is called
outside from lock protection while 'stop' flag is a volatile
shared variable so cannot be changed when not in lock.

Note that this bugs fires ONLY when we use more then 2 threads,
so commonly only in a QUAD or OCTAL machine.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>
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1 parent 2b740f5 commit 8eae6a95fb3b446e2782be12aff519164e521e4d @mcostalba committed Feb 14, 2010
Showing with 10 additions and 9 deletions.
  1. +10 −9 src/search.cpp
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19 src/search.cpp
@@ -81,7 +81,7 @@ namespace {
void get_beta_counters(Color us, int64_t& our, int64_t& their) const;
bool idle_thread_exists(int master) const;
bool thread_is_available(int slave, int master) const;
- bool thread_should_stop(int threadID);
+ bool thread_should_stop(int threadID) const;
void wake_sleeping_threads();
void put_threads_to_sleep();
void idle_loop(int threadID, SplitPoint* waitSp);
@@ -1881,10 +1881,12 @@ namespace {
/* Here we have the lock still grabbed */
// If this is the master thread and we have been asked to stop because of
- // a beta cutoff higher up in the tree, stop all slave threads.
+ // a beta cutoff higher up in the tree, stop all slave threads. Note that
+ // thread_should_stop(threadID) does not imply that 'stop' flag is set, so
+ // do this explicitly now, under lock protection.
if (sp->master == threadID && TM.thread_should_stop(threadID))
for (int i = 0; i < TM.active_threads(); i++)
- if (sp->slaves[i])
+ if (sp->slaves[i] || i == threadID)
TM.set_stop_request(i);
sp->cpus--;
@@ -2016,10 +2018,12 @@ namespace {
/* Here we have the lock still grabbed */
// If this is the master thread and we have been asked to stop because of
- // a beta cutoff higher up in the tree, stop all slave threads.
+ // a beta cutoff higher up in the tree, stop all slave threads. Note that
+ // thread_should_stop(threadID) does not imply that 'stop' flag is set, so
+ // do this explicitly now, under lock protection.
if (sp->master == threadID && TM.thread_should_stop(threadID))
for (int i = 0; i < TM.active_threads(); i++)
- if (sp->slaves[i])
+ if (sp->slaves[i] || i == threadID)
TM.set_stop_request(i);
sp->cpus--;
@@ -2764,7 +2768,7 @@ namespace {
// cutoff has occurred in the thread's currently active split point, or in
// some ancestor of the current split point.
- bool ThreadsManager::thread_should_stop(int threadID) {
+ bool ThreadsManager::thread_should_stop(int threadID) const {
assert(threadID >= 0 && threadID < ActiveThreads);
@@ -2778,10 +2782,7 @@ namespace {
for (sp = threads[threadID].splitPoint; sp != NULL; sp = sp->parent)
if (sp->finished)
- {
- threads[threadID].stop = true;
return true;
- }
return false;
}

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