On Sunday 26 August 2012 01:30:43 David Faure wrote:
> On Saturday 25 August 2012 22:43:58 Julian Seward wrote:
> > Or maybe Qt really is racey
>
> Bingo :)
OK, maybe not. Turns out the code runs as intended by the Qt developers.
Marc Mutz said "
The standard says it's racy, but the implementation of
std::atomic::load(memory_order_acquire) won't look different. Simple reads
and writes on x86 are already sequentially consistent. Think MESI cache
coherency. Before a CPU writes to a memory location it needs to acquire
exclusive ownership (E) of the cache line, the well-known "hardware mutex" on
a cache line that produces False Sharing, too. This seems to hold for all
architectures, cf. threads re: "Brief example ..." at
http://www.decadent.org.uk/pipermail/cpp-threads/2008-December/thread.html
"
I attached a pure C++ testcase of the issue. Compile it with
g++ testcase_atomic_ops_helgrind.cpp -o testcase_atomic_ops_helgrind -lpthread
Thiago expects that helgrind can't autodetect this case and that helgrind-
macros markup is needed in Qt, I'm fine with adding that if you guys agree --
after you show me how by modifying the attached example :-)
Thanks.
--
David Faure, fa...@kde.org, http://www.davidfaure.fr
Sponsored by Nokia to work on KDE, incl. KDE Frameworks 5
#include <pthread.h>
template <typename T> struct QAtomicOps // qgenericatomic.h
{
static inline
T loadAcquire(const T &_q_value)
{
T tmp = *static_cast<const volatile T *>(&_q_value);
return tmp;
}
static inline
void storeRelease(T &_q_value, T newValue)
{
*static_cast<volatile T *>(&_q_value) = newValue;
}
};
class QBasicAtomicInt // qbasicatomic.h
{
public:
typedef QAtomicOps<int> Ops;
int _q_value;
int loadAcquire() const { return Ops::loadAcquire(_q_value); }
void storeRelease(int newValue) { Ops::storeRelease(_q_value, newValue); }
};
// Modelled after qt_metatype_id()
static int onDemandNumber() {
static QBasicAtomicInt sharedInt = { 0 };
if (!sharedInt.loadAcquire()) {
// some calculation goes here
sharedInt.storeRelease(42);
}
return sharedInt.loadAcquire();
}
void * threadStart(void *)
{
onDemandNumber();
onDemandNumber();
onDemandNumber();
onDemandNumber();
return 0;
}
int main( int argc, char** argv ) {
pthread_t thread1;
if ( pthread_create(&thread1, 0, threadStart, 0) )
return 1;
pthread_t thread2;
if ( pthread_create(&thread2, 0, threadStart, 0) )
return 1;
void* v;
pthread_join(thread1, &v);
pthread_join(thread2, &v);
return 0;
}
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