Fix race in ThreadTaskTag

Assume T0 calls ThreadPool::pushTask twice, then ThreadPool::waitUntilDone:

T0 calls ThreadTaskTag::onTaskPushed:
mnTasksWorking = 1, maTasksComplete.reset

T1 runs the 1st task to completion and calls ThreadTaskTag::onTaskWorkerDone:
mnTasksWorking = 0; suspended...

T0 calls ThreadTaskTag::onTaskPushed:
mnTaskWorking = 1, maTasksComplete.reset

T1 continues in the call to ThreadTaskTag::onTaskWorkerDone:
..., maTasksComplete.set

T0 calls ThreadTaskTag::waitUntilDone and immediately returns

T2 only now starts to run the 2nd task

Change-Id: Ic29101a4791fca2a1a4d54b559f10ff706e8a20d
(cherry picked from commit d36b3bcb7e08f7f73709b7afe9b8f9ec22a0fcd2)
Reviewed-on: https://gerrit.libreoffice.org/31485
Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

1 files changed, 10 insertions, 8 deletions

diff --git a/comphelper/source/misc/threadpool.cxx b/comphelper/source/misc/threadpool.cxx
index f399274eed24..286fbf697358 100644
--- a/comphelper/source/misc/threadpool.cxx
+++ b/comphelper/source/misc/threadpool.cxx
@@ -26,8 +26,9 @@ static thread_local bool gbIsWorkerThread;
 // used to group thread-tasks for waiting in waitTillDone()
 class COMPHELPER_DLLPUBLIC ThreadTaskTag
 {
-    oslInterlockedCount  mnTasksWorking;
-    osl::Condition       maTasksComplete;
+    osl::Mutex mMutex;
+    std::size_t mnTasksWorking;
+    osl::Condition maTasksComplete;
 
 public:
     ThreadTaskTag();
@@ -305,17 +306,18 @@ ThreadTaskTag::ThreadTaskTag() : mnTasksWorking(0)
 
 void ThreadTaskTag::onTaskPushed()
 {
-    oslInterlockedCount n = osl_atomic_increment(&mnTasksWorking);
-    assert( n < 65536 ); // sanity checking
-    (void)n; // avoid -Wunused-variable in release build
+    osl::MutexGuard g(mMutex);
+    assert( mnTasksWorking < 65535 ); // sanity checking
+    ++mnTasksWorking;
     maTasksComplete.reset();
 }
 
 void ThreadTaskTag::onTaskWorkerDone()
 {
-    sal_Int32 nCount = osl_atomic_decrement(&mnTasksWorking);
-    assert(nCount >= 0);
-    if (nCount == 0)
+    osl::MutexGuard g(mMutex);
+    assert(mnTasksWorking > 0);
+    --mnTasksWorking;
+    if (mnTasksWorking == 0)
         maTasksComplete.set();
 }