Re: [musl] Re: MT fork and key_lock in pthread_key_create.c

[Rich Felker <dalias@libc.org>]

On Thu, Oct 06, 2022 at 10:02:11AM +0300, Alexey Izbyshev wrote:
> On 2022-10-06 09:37, Alexey Izbyshev wrote:
> >Hi,
> >
> >I noticed that fork() doesn't take key_lock that is used to protect
> >the global table of thread-specific keys. I couldn't find mentions of
> >this lock in the MT fork discussion in the mailing list archive. Was
> >this lock overlooked?
> >
> >Also, I looked at how __aio_atfork() handles a similar case with
> >maplock, and it seems wrong. It takes the read lock and then simply
> >unlocks it both in the parent and in the child. But if there were
> >other holders of the read lock at the time of fork(), the lock won't
> >end up in the unlocked state in the child. It should probably be
> >completely nulled-out in the child instead.
> >
> Looking at aio further, I don't understand how it's supposed to work
> with MT fork at all. __aio_atfork() is called in _Fork() when the
> allocator locks are already held. Meanwhile another thread could be
> stuck in __aio_get_queue() holding maplock in exclusive mode while
> trying to allocate, resulting in deadlock.

Indeed, this is messy and I don't think it makes sense to be doing
this at all. The child is just going to throw away the state so the
parent shouldn't need to synchronize at all, but if we walk the
multi-level map[] table in the child after async fork, it's possible
that the contents seen are inconsistent, even that the pointers are
only half-written or something.

I see a few possible solutions:

1. Just set map = 0 in the child and leak the memory. This is not
   going to matter unless you're doing multiple generations of fork
   with aio anyway.

2. The same, but be a little bit smarter. pthread_rwlock_tryrdlock in
   the child, and if it succeeds, we know the map is consistent so we
   can just zero it out the same as now. Still "leaks" but only on
   contention to expand the map.

3. Getting a little smarter still: move the __aio_atfork for the
   parent side from _Fork to fork, outside of the critical section
   where malloc lock is held. Then proceed as in (2). Now, the
   tryrdlock is guaranteed to succeed in the child. Leak is only
   possible when _Fork is used (in which case the child context is an
   async signal one, and thus calling any aio_* that would allocate
   map[] again is UB -- note that in this case, the only reason we
   have to do anything at all in the child is to prevent close from
   interacting with aio).

After writing them out, 3 seems like the right choice.

Rich