[BUG] completely bonkers use of set_need_resched + VM_FAULT_NOPAGE

[Peter Zijlstra]

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).