dvyukov 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);
----------------
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.
https://reviews.llvm.org/D50549
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