Optimized implementation of QReadWriteLock

QReadWriteLock is supposed to be a better alternative to QMutex when there
are only a few writers but potentially lots of reads. However, in practice
the previous implementation was much slower, unless you really do a lot of
work with the lock for read and you have lots of contention.
Indeed, the previous implementation was locking a QMutex both for lock, and
unlock (making it already at least twice as slow as QMutex).

This new implementation brings QReadWriteLock back to the same level as QMutex:
 - No memory allocations in the uncontended case (almost no overhead allowing to
   create many of them in classes)
 - Lock-free if there is no contention

Should support up to 2^31 concurrent readers on 64 bit platforms, and 2^28
on 32 bit platforms

Change-Id: Ifa2fc999075cbb971088f4ee8e6fde78ce262da3
Reviewed-by: Edward Welbourne <edward.welbourne@theqtcompany.com>
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>
Reviewed-by: Robin Burchell <robin.burchell@viroteck.net>

1 files changed, 6 insertions, 4 deletions

diff --git a/src/corelib/thread/qwaitcondition_unix.cpp b/src/corelib/thread/qwaitcondition_unix.cpp
index 2a98fd2897..6adee5412e 100644
--- a/src/corelib/thread/qwaitcondition_unix.cpp
+++ b/src/corelib/thread/qwaitcondition_unix.cpp
@@ -221,9 +221,12 @@ bool QWaitCondition::wait(QMutex *mutex, unsigned long time)
 
 bool QWaitCondition::wait(QReadWriteLock *readWriteLock, unsigned long time)
 {
-    if (!readWriteLock || readWriteLock->d->accessCount == 0)
+    if (!readWriteLock)
         return false;
-    if (readWriteLock->d->accessCount < -1) {
+    auto previousState = readWriteLock->stateForWaitCondition();
+    if (previousState == QReadWriteLock::Unlocked)
+        return false;
+    if (previousState == QReadWriteLock::RecursivelyLocked) {
         qWarning("QWaitCondition: cannot wait on QReadWriteLocks with recursive lockForWrite()");
         return false;
     }
@@ -231,12 +234,11 @@ bool QWaitCondition::wait(QReadWriteLock *readWriteLock, unsigned long time)
     report_error(pthread_mutex_lock(&d->mutex), "QWaitCondition::wait()", "mutex lock");
     ++d->waiters;
 
-    int previousAccessCount = readWriteLock->d->accessCount;
     readWriteLock->unlock();
 
     bool returnValue = d->wait(time);
 
-    if (previousAccessCount < 0)
+    if (previousState == QReadWriteLock::LockedForWrite)
         readWriteLock->lockForWrite();
     else
         readWriteLock->lockForRead();