On Fri, 3 May 2024 at 14:24, Al Viro <v...@zeniv.linux.org.uk> wrote:
>
> Can we get to ep_item_poll(epi, ...) after eventpoll_release_file()
> got past __ep_remove()? Because if we can, we have a worse problem -
> epi freed under us.
Look at the hack in __ep_remove(): if it is concurrent with
eventpoll_release_file(), it will hit this code
spin_lock(&file->f_lock);
if (epi->dying && !force) {
spin_unlock(&file->f_lock);
return false;
}
and not free the epi.
But as far as I can tell, almost nothing else cares about the f_lock
and dying logic.
And in fact, I don't think doing
spin_lock(&file->f_lock);
is even valid in the places that look up file through "epi->ffd.file",
because the lock itself is inside the thing that you can't trust until
you've taken the lock...
So I agree with Kees about the use of "atomic_dec_not_zero()" kind of
logic - but it also needs to be in an RCU-readlocked region, I think.
I wish epoll() just took the damn file ref itself. But since it relies
on the file refcount to release the data structure, that obviously
can't work.
Linus
