From 76c3eee4020cae681857ee17406c655f61f9082c Mon Sep 17 00:00:00 2001
From: Mike Achtelik <mike.achtelik@gmail.com>
Date: Wed, 17 Jun 2020 08:04:03 +0200
Subject: [PATCH] Remove pthread storage for thread local data

The thread specific QThreadData is currently referenced as a thread_local
variable and using a thread specific value via pthread_setspecific.

Having both is not necessary, as the pthread value is never directly accessed
and only used in the pthread destructor. Using a holder, we can achieve the same
and get rid of the pthread handling altogether.

This also fixes a bug, where the thread_local currentThreadData is already null,
when entering the pthread destructor. In this case it would lead to a new QThreadData
being created, when finishing an adopted thread.

Pick-to: 5.15
Change-Id: Ib23b840f804637e6b7cebd89016672a34a575380
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
---
 src/corelib/thread/qthread_unix.cpp | 82 ++++++++---------------------
 1 file changed, 22 insertions(+), 60 deletions(-)

diff --git a/src/corelib/thread/qthread_unix.cpp b/src/corelib/thread/qthread_unix.cpp
index 35bfd35f1a3..18b6caf5fc9 100644
--- a/src/corelib/thread/qthread_unix.cpp
+++ b/src/corelib/thread/qthread_unix.cpp
@@ -109,81 +109,43 @@ static_assert(sizeof(pthread_t) <= sizeof(Qt::HANDLE));
 enum { ThreadPriorityResetFlag = 0x80000000 };
 
 
-static thread_local QThreadData *currentThreadData = nullptr;
-
-static pthread_once_t current_thread_data_once = PTHREAD_ONCE_INIT;
-static pthread_key_t current_thread_data_key;
-
-static void destroy_current_thread_data(void *p)
+class QThreadDataHolder
 {
-#if defined(Q_OS_VXWORKS)
-    // Calling setspecific(..., 0) sets the value to 0 for ALL threads.
-    // The 'set to 1' workaround adds a bit of an overhead though,
-    // since this function is called twice now.
-    if (p == (void *)1)
-        return;
-#endif
-    // POSIX says the value in our key is set to zero before calling
-    // this destructor function, so we need to set it back to the
-    // right value...
-    pthread_setspecific(current_thread_data_key, p);
-    QThreadData *data = static_cast<QThreadData *>(p);
-    if (data->isAdopted) {
-        QThread *thread = data->thread.loadAcquire();
-        Q_ASSERT(thread);
-        QThreadPrivate *thread_p = static_cast<QThreadPrivate *>(QObjectPrivate::get(thread));
-        Q_ASSERT(!thread_p->finished);
-        thread_p->finish(thread);
+public:
+    QThreadData* data;
+
+    ~QThreadDataHolder()
+    {
+        if (!data)
+            return;
+
+        if (data->isAdopted) {
+            QThread *thread = data->thread.loadAcquire();
+            Q_ASSERT(thread);
+            QThreadPrivate *thread_p = static_cast<QThreadPrivate *>(QObjectPrivate::get(thread));
+            Q_ASSERT(!thread_p->finished);
+            thread_p->finish(thread);
+        }
+        data->deref();
     }
-    data->deref();
-
-    // ... but we must reset it to zero before returning so we aren't
-    // called again (POSIX allows implementations to call destructor
-    // functions repeatedly until all values are zero)
-    pthread_setspecific(current_thread_data_key,
-#if defined(Q_OS_VXWORKS)
-                                                 (void *)1);
-#else
-                                                 nullptr);
-#endif
-}
-
-static void create_current_thread_data_key()
-{
-    pthread_key_create(&current_thread_data_key, destroy_current_thread_data);
-}
-
-static void destroy_current_thread_data_key()
-{
-    pthread_once(&current_thread_data_once, create_current_thread_data_key);
-    pthread_key_delete(current_thread_data_key);
-
-    // Reset current_thread_data_once in case we end up recreating
-    // the thread-data in the rare case of QObject construction
-    // after destroying the QThreadData.
-    pthread_once_t pthread_once_init = PTHREAD_ONCE_INIT;
-    current_thread_data_once = pthread_once_init;
-}
-Q_DESTRUCTOR_FUNCTION(destroy_current_thread_data_key)
+};
 
+static thread_local QThreadDataHolder currentThreadData;
 
 // Utility functions for getting, setting and clearing thread specific data.
 static QThreadData *get_thread_data()
 {
-    return currentThreadData;
+    return currentThreadData.data;
 }
 
 static void set_thread_data(QThreadData *data)
 {
-    currentThreadData = data;
-    pthread_once(&current_thread_data_once, create_current_thread_data_key);
-    pthread_setspecific(current_thread_data_key, data);
+    currentThreadData.data = data;
 }
 
 static void clear_thread_data()
 {
-    currentThreadData = nullptr;
-    pthread_setspecific(current_thread_data_key, nullptr);
+    currentThreadData.data = nullptr;
 }
 
 template <typename T>