Re: [patch 0/6] lightweight robust futexes: -V3 - Why in userspace?

Ingo Molnar
2006-02-16T23:41:44Z

* Esben Nielsen <simlo@phys.au.dk> wrote:

> > this is racy - we cannot know whether the PID wrapped around.
> >
> What about adding more bits to check on? The PID to lookup the task_t
> and then some extra bits to uniquely identify the actual task.

which would just be a fancy name for a wider PID space, and would thus
still not protect against PID reuse :-)

> > nor does this method offer any solution for the case where there are
> > already waiters pending: they might be hung forever.
>
> It was for this case I suggested maintaining a list of waiters within
> the kernel on each task_t. The adding has to be done FUTEX_WAIT so the
> adding operation needs to be protected.

i'm not sure i follow - what list is this and how would it be
maintained?

> > With our solution
> > one of those waiters gets woken up and notice that the lock is dead.
> > (and in the unlikely even of that thread dying too while trying to
> > recover the data, the kernel will do yet another wakeup, of the next
> > waiter.)
> >
> I admit your solution is a good one. The only drawback - besides being
> untraditional - is that memory corruption can leave futexes locked at
> exit.

so? Memory corruption can overwrite the futex value anyway, and can thus
cause the wrong owner to be identified - causing a locked futex. This
patch does not protect against bad effects of memory corruption -
there's really no way to keep userspace from breaking itself.

Ingo