Fix futexNeedsWake() on 64-bit systems

It was mistaking semaphore values over 0 as waiters, regardless of
actual waiters.

Pick-to: 6.1
Change-Id: Icebd01592ca8bdc1687a29dc569e3b630a262606
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

1 files changed, 5 insertions, 4 deletions

diff --git a/src/corelib/thread/qsemaphore.cpp b/src/corelib/thread/qsemaphore.cpp
index 45983d47ab..213852250f 100644
--- a/src/corelib/thread/qsemaphore.cpp
+++ b/src/corelib/thread/qsemaphore.cpp
@@ -150,10 +150,11 @@ static int futexAvailCounter(quintptr v)
 
 static bool futexNeedsWake(quintptr v)
 {
-    // If we're counting waiters, the number of waiters is stored in the low 31
-    // bits of the high word (that is, bits 32-62). If we're not, then we use
-    // bit 31 to indicate anyone is waiting. Either way, if any bit 31 or above
-    // is set, there are waiters.
+    // If we're counting waiters, the number of waiters plus value is stored in the
+    // low 31 bits of the high word (that is, bits 32-62). If we're not, then we only
+    // use futexNeedsWakeAllBit to indicate anyone is waiting.
+    if constexpr (futexHasWaiterCount)
+        return (v >> 32) > (unsigned(v));
     return v >> 31;
 }