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QThread::terminate(): don't depend on stack unwinding

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Posix doesn't seem to specify whether the stack of cancelled threads
is unwound, and there's nothing preventing a QThread from
terminate()ing itself, so be extra careful to drop the mutex before
calling pthread_cancel.

We can't drop the mutex in general, as that would open a window for
the following race condition:

   T1                       T2
   t3->terminate()
     lock();
     read ID;
     terminated = true;
     unlock();
   ----------- t3 exits naturally -----------
                            t3->wait();
                            t4->start(); // gets ex-t3's ID
     pthread_cancel(ID) // oops, cancels new t4

But we can drop it when this == currentThread(), because said window
does not exist: While this_thread is executing terminate(), it cannot
at the same time exit naturally.

As drive-by, scope a variable tighter.

Pick-to: 6.8 6.7 6.5
Change-Id: I77a628e62d88e383d5aa91cfd97440186c997fc4
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
This commit is contained in:
Marc Mutz 2024-01-11 11:09:26 +01:00
parent 6f70ab027e
commit 272c0215c2
1 changed files with 14 additions and 3 deletions
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17
src/corelib/thread/qthread_unix.cpp
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@ -751,7 +751,8 @@ void QThread::terminate()
Q_D(QThread);
QMutexLocker locker(&d->mutex);
if (!d->data->threadId.loadRelaxed())
const auto id = d->data->threadId.loadRelaxed();
if (!id)
return;
if (d->terminated) // don't try again, avoids killing the wrong thread on threadId reuse (ABA)
@ -759,8 +760,18 @@ void QThread::terminate()
d->terminated = true;
int code = pthread_cancel(from_HANDLE<pthread_t>(d->data->threadId.loadRelaxed()));
if (code) {
const bool selfCancelling = d->data == currentThreadData;
if (selfCancelling) {
// Posix doesn't seem to specify whether the stack of cancelled threads
// is unwound, and there's nothing preventing a QThread from
// terminate()ing itself, so drop the mutex before calling
// pthread_cancel():
locker.unlock();
}
if (int code = pthread_cancel(from_HANDLE<pthread_t>(id))) {
if (selfCancelling)
locker.relock();
d->terminated = false; // allow to try again
qErrnoWarning(code, "QThread::start: Thread termination error");
}