Re: [PATCH 3/3] eventfd: add internal reference counting to fix notifier race conditions

Davide Libenzi
2009-06-19T21:33:15Z

On Fri, 19 Jun 2009, Gregory Haskins wrote:

> Davide Libenzi wrote:
> > On Fri, 19 Jun 2009, Gregory Haskins wrote:
> >
> >
> >> eventfd currently emits a POLLHUP wakeup on f_ops->release() to generate a
> >> notifier->release() callback. This lets notification clients know if
> >> the eventfd is about to go away and is very useful particularly for
> >> in-kernel clients. However, as it stands today it is not possible to
> >> use the notification API in a race-free way. This patch adds some
> >> additional logic to the notification subsystem to rectify this problem.
> >>
> >> Background:
> >> -----------------------
> >> Eventfd currently only has one reference count mechanism: fget/fput. This
> >> in of itself is normally fine. However, if a client expects to be
> >> notified if the eventfd is closed, it cannot hold a fget() reference
> >> itself or the underlying f_ops->release() callback will never be invoked
> >> by VFS. Therefore we have this somewhat unusual situation where we may
> >> hold a pointer to an eventfd object (by virtue of having a waiter registered
> >> in its wait-queue), but no reference. This makes it nearly impossible to
> >> design a mutual decoupling algorithm: you cannot unhook one side from the
> >> other (or vice versa) without racing.
> >>
> >
> > And why is that?
> >
> > struct xxx {
> > struct mutex mtx;
> > struct file *file;
> > ...
> > };
> >
> > struct file *xxx_get_file(struct xxx *x) {
> > struct file *file;
> >
> > mutex_lock(&x->mtx);
> > file = x->file;
> > if (!file)
> > mutex_unlock(&x->mtx);
> > return file;
> > }
> >
> > void xxx_release_file(struct xxx *x) {
> > mutex_unlock(&x->mtx);
> > }
> >
> > void handle_POLLHUP(struct xxx *x) {
> > struct file *file;
> >
> > file = xxx_get_file(x);
> > if (file) {
> > unhook_waitqueue(file, ...);
> > x->file = NULL;
> > xxx_release_file(x);
> > }
> > }
> >
> >
> > Every time you need to "use" file, you call xxx_get_file(), and if you get
> > NULL, it means it's gone and you handle it accordigly to your IRQ fd
> > policies. As soon as you done with the file, you call xxx_release_file().
> > Replace "mtx" with the lock that fits your needs.
> >
>
> Consider what would happen if the f_ops->release() was preempted inside
> the wake_up_locked_polled() after it dereferenced the xxx from the list,
> but before it calls the callback(POLLHUP). The xxx object, and/or the
> .text for the xxx object may be long gone by the time it comes back
> around. Afaict, there is no way to guard against that scenario unless
> you do something like 2/3+3/3. Or am I missing something?

Right. Don't you see an easier answer to that, instead of adding 300 lines
of code to eventfd?
For example, turning wake_up_locked() into a nornal wake_up().

- Davide