Re: [RFC-PATCH 1/2] mm: Add __GFP_NO_LOCKS flag

Michal Hocko
2020-08-13T13:46:14Z

On Thu 13-08-20 15:27:15, Thomas Gleixner wrote:
> Michal Hocko <mhocko@suse.com> writes:
> > On Thu 13-08-20 11:58:40, Uladzislau Rezki wrote:
> > [...]
> >> Sorry for jumping in. We can rely on preemptable() for sure, if CONFIG_PREEMPT_RT
> >> is enabled, something like below:
> >>
> >> if (IS_ENABLED_RT && preemptebale())
> >
> > Sure. I thought this was an RT specific thing that would be noop
> > otherwise.
>
> Well, even if RT specific it would be still something returning either
> true or false unconditionally.
>
> And guarding it with RT is not working either because then you are back
> to square one with the problem which triggered the discussion in the
> first place:
>
> raw_spin_lock()
> alloc()
> if (RT && !preemptible()) <- False because RT == false
> goto bail;
>
> spin_lock(&zone->lock) --> LOCKDEP complains
>
> So either you convince Paul not to do that or you need to do something
> like I suggested in my other reply.

Can we somehow annotate the lock to be safe for nesting for lockdep?
--
Michal Hocko
SUSE Labs