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:
> > > > 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;`
https://reviews.llvm.org/D50549
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