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Make the QEventDispatcherWin32Private::interrupt flag atomic

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Not entirely sure that this solves the problem reported in the bug
report, but here's the theory: the loop

 633        while (!d->interrupt) {
 ...
 710        }

has few calls that recurse back, so the compiler optimizer can assume
that the variable remains unchanged (not interrupted) in most of the
branches. Additionally, it can assume the variable did not change from
there to

 712        // still nothing - wait for message or signalled objects
 713        canWait = (!retVal
 714                   && !d->interrupt
 715                   && (flags & QEventLoop::WaitForMoreEvents));

Which causes canWait to be true, despite having been interrupted by
another thread.

Changing to an atomic does not force the reloading of the variable
(strictly speaking, would need volatile, but all atomic implementations
do reload now), but it solves the problem of data race, which was UB.

The equivalent variable in the Unix event dispatcher is atomic (commit
49d7e71f77f899c05e4b5187e8834dfcbddf4505 from 2013). I've reordered the
bool members so they're all together and reduce the amount of padding in
this class.

Fixes: QTBUG-72438
Change-Id: I4ac1156702324f0fb814fffd156f290df94dc4c7
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
This commit is contained in:
Thiago Macieira 2018-12-10 19:18:57 -08:00
parent 1500d2283e
commit 197029b3d2
2 changed files with 9 additions and 8 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

10
src/corelib/kernel/qeventdispatcher_win.cpp
View File

@ -95,7 +95,7 @@ class QEventDispatcherWin32Private;
LRESULT QT_WIN_CALLBACK qt_internal_proc(HWND hwnd, UINT message, WPARAM wp, LPARAM lp); LRESULT QT_WIN_CALLBACK qt_internal_proc(HWND hwnd, UINT message, WPARAM wp, LPARAM lp);
QEventDispatcherWin32Private::QEventDispatcherWin32Private() QEventDispatcherWin32Private::QEventDispatcherWin32Private()
: threadId(GetCurrentThreadId()), interrupt(false), closingDown(false), internalHwnd(0), : threadId(GetCurrentThreadId()), interrupt(false), internalHwnd(0),
getMessageHook(0), serialNumber(0), lastSerialNumber(0), sendPostedEventsWindowsTimerId(0), getMessageHook(0), serialNumber(0), lastSerialNumber(0), sendPostedEventsWindowsTimerId(0),
wakeUps(0), activateNotifiersPosted(false), winEventNotifierActivatedEvent(NULL) wakeUps(0), activateNotifiersPosted(false), winEventNotifierActivatedEvent(NULL)
{ {
@ -552,7 +552,7 @@ bool QEventDispatcherWin32::processEvents(QEventLoop::ProcessEventsFlags flags)
wakeUp(); // trigger a call to sendPostedEvents() wakeUp(); // trigger a call to sendPostedEvents()
} }
d->interrupt = false; d->interrupt.store(false);
emit awake(); emit awake();
bool canWait; bool canWait;
@ -568,7 +568,7 @@ bool QEventDispatcherWin32::processEvents(QEventLoop::ProcessEventsFlags flags)
pHandles = &d->winEventNotifierActivatedEvent; pHandles = &d->winEventNotifierActivatedEvent;
} }
QVarLengthArray<MSG> processedTimers; QVarLengthArray<MSG> processedTimers;
while (!d->interrupt) { while (!d->interrupt.load()) {
MSG msg; MSG msg;
bool haveMessage; bool haveMessage;
@ -649,7 +649,7 @@ bool QEventDispatcherWin32::processEvents(QEventLoop::ProcessEventsFlags flags)
// still nothing - wait for message or signalled objects // still nothing - wait for message or signalled objects
canWait = (!retVal canWait = (!retVal
&& !d->interrupt && !d->interrupt.load()
&& (flags & QEventLoop::WaitForMoreEvents)); && (flags & QEventLoop::WaitForMoreEvents));
if (canWait) { if (canWait) {
emit aboutToBlock(); emit aboutToBlock();
@ -1016,7 +1016,7 @@ void QEventDispatcherWin32::wakeUp()
void QEventDispatcherWin32::interrupt() void QEventDispatcherWin32::interrupt()
{ {
Q_D(QEventDispatcherWin32); Q_D(QEventDispatcherWin32);
d->interrupt = true; d->interrupt.store(true);
wakeUp(); wakeUp();
} }

7
src/corelib/kernel/qeventdispatcher_win_p.h
View File

@ -165,8 +165,7 @@ public:
DWORD threadId; DWORD threadId;
bool interrupt; QAtomicInt interrupt;
bool closingDown;
// internal window handle used for socketnotifiers/timers/etc // internal window handle used for socketnotifiers/timers/etc
HWND internalHwnd; HWND internalHwnd;
@ -193,9 +192,11 @@ public:
void postActivateSocketNotifiers(); void postActivateSocketNotifiers();
void doWsaAsyncSelect(int socket, long event); void doWsaAsyncSelect(int socket, long event);
bool closingDown = false;
bool winEventNotifierListModified = false;
HANDLE winEventNotifierActivatedEvent; HANDLE winEventNotifierActivatedEvent;
QList<QWinEventNotifier *> winEventNotifierList; QList<QWinEventNotifier *> winEventNotifierList;
bool winEventNotifierListModified = false;
void activateEventNotifier(QWinEventNotifier * wen); void activateEventNotifier(QWinEventNotifier * wen);
QList<MSG> queuedUserInputEvents; QList<MSG> queuedUserInputEvents;