BillyONeal added inline comments.
================
Comment at:
test/std/thread/thread.threads/thread.thread.class/thread.thread.member/detach.pass.cpp:73
assert(!t0.joinable());
while (!done) {}
assert(G::op_run);
----------------
dvyukov wrote:
> BillyONeal wrote:
> > BillyONeal wrote:
> > > dvyukov wrote:
> > > > BillyONeal wrote:
> > > > > BillyONeal wrote:
> > > > > > dvyukov wrote:
> > > > > > > I don't immediately see how the race on n_alive/op_run happens.
> > > > > > > It seems that the modifications in the thread happen before this
> > > > > > > line, and modifications in main happen after this line. How can
> > > > > > > both of them modify the variables at the same time?
> > > > > > The destruction of g here races with the destruction of the
> > > > > > DECAY_COPY'd copy of G used as the parameter of operator(). That
> > > > > > is, line 69 creates a copy of g, passes that to the started thread,
> > > > > > the started thread calls gCopy(). gCopy() doesn't return until the
> > > > > > done flag is set, but the destruction of the object on which op()
> > > > > > is being called is not so synchronized. Most of the other thread
> > > > > > tests avoid this problem by joining with the thread; joining waits
> > > > > > for the destruction of the DECAY_COPY'd parameters, but this does
> > > > > > not.
> > > > > >
> > > > > > (This is one of the reasons detach() should basically never be
> > > > > > called anywhere)
> > > > > >
> > > > > (That is to say, there's nothing to prevent both threads from
> > > > > executing G::!G() on the two different copies of g... making op_run
> > > > > atomic is probably avoidable but I'm being paranoid given that there
> > > > > was already thread unsafety here...)
> > > > What is gCopy? I don't see anything named gCopy in this file...
> > > >
> > > > Do we care about completion of destruction? Why? We wait for done to be
> > > > set, and other variables are already updated at that point. Why does it
> > > > matter that "the destruction of the object on which op() is being
> > > > called is not so synchronized."?
> > > Because the destructor does `--n_alive;`
> > >What is gCopy? I don't see anything named gCopy in this file...
> >
> > The copy is made in the constructor of std::thread. std::thread makes a
> > copy of all the input parameters, gives the copy to the started thread, and
> > then std::invoke is called there.
> >
> > >Why does it matter that "the destruction of the object on which op() is
> > >being called is not so synchronized."?
> >
> > Because the two dtors race on `--n_alive;` when `n_alive` is not atomic.
> But the first dtor runs before "while (!done) {}" line and the second dtor
> runs after "while (!done) {}" line, no?
> Or there is third object involved? But then I don't see how joining the
> thread would help either.
>But the first dtor runs before "while (!done) {}" line
No, both dtors are run after the while (!done) {} line. The first dtor runs on
line 76 (when the local variable g is destroyed), and the second dtor runs
after operator() returns in the constructed thread. The constructed thread is
morally doing:
```
void threadproc(G * g) {
g->operator(); // setting done happens in here
delete g; // dtor of second copy runs here
}
```
> I don't see how joining the thread would help either.
Joining with the thread would wait for the second dtor -- the one after op()
returns -- to complete. Of course joining with the thread isn't doable here
given that the point is to test thread::detach :)
https://reviews.llvm.org/D50549
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