[BUG] completely bonkers use of set_need_resched + VM_FAULT_NOPAGE

[Peter Zijlstra]

On Fri, Sep 13, 2013 at 10:41:54AM +0200, Daniel Vetter wrote:
> On Fri, Sep 13, 2013 at 10:29 AM, Peter Zijlstra <peterz at infradead.org> wrote:
> > On Fri, Sep 13, 2013 at 09:46:03AM +0200, Thomas Hellstrom wrote:
> >> >>if (!bo_tryreserve()) {
> >> >>     up_read mmap_sem(); // Release the mmap_sem to avoid deadlocks.
> >> >>     bo_reserve();               // Wait for the BO to become available (interruptible)
> >> >>     bo_unreserve();           // Where is bo_wait_unreserved() when we need it, Maarten :P
> >> >>     return VM_FAULT_RETRY; // Go ahead and retry the VMA walk, after regrabbing
> >> >>}
> >>
> >> Anyway, could you describe what is wrong, with the above solution, because
> >> it seems perfectly legal to me.
> >
> > Luckily the rule of law doesn't have anything to do with this stuff --
> > at least I sincerely hope so.
> >
> > The thing that's wrong with that pattern is that its still not
> > deterministic - although its a lot better than the pure trylock. Because
> > you have to release and re-acquire with the trylock another user might
> > have gotten in again. Its utterly prone to starvation.
> >
> > The acquire+release does remove the dead/life-lock scenario from the
> > FIFO case, since blocking on the acquire will allow the other task to
> > run (or even get boosted on -rt).
> >
> > Aside from that there's nothing particularly wrong with it and lockdep
> > should be happy afaict (but I haven't had my morning juice yet).
> 
> bo_reserve internally maps to a ww-mutex and task can already hold
> ww-mutex (potentially even the same for especially nasty userspace).

OK, yes I wasn't aware of that. Yes in that case you're quite right.