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

Paul E. McKenney
2020-08-11T16:03:28Z

On Tue, Aug 11, 2020 at 05:43:16PM +0200, Thomas Gleixner wrote:
> "Paul E. McKenney" <paulmck@kernel.org> writes:
> > On Tue, Aug 11, 2020 at 04:44:21PM +0200, Thomas Gleixner wrote:
> >> Now RCU creates a new thing which enforces to make page allocation in
> >> atomic context possible on RT. What for?
> >>
> >> What's the actual use case in truly atomic context for this new thing on
> >> an RT kernel?
> >
> > It is not just RT kernels. CONFIG_PROVE_RAW_LOCK_NESTING=y propagates
> > this constraint to all configurations, and a patch in your new favorite
> > subsystem really did trigger this lockdep check in a non-RT kernel.
> >
> >> The actual RCU code disabling interrupts is an implementation detail
> >> which can easily be mitigated with a local lock.
> >
> > In this case, we are in raw-spinlock context on entry to kfree_rcu().
>
> Where?

Some BPF code that needs to process and free a list. As noted above,
this is a patch rather than something that is already in mainline.
Not surprising, though, given call_rcu() invocations in similar contexts.

Yes, we can perhaps rework all current and future callers to avoid
invoking both call_rcu() and kfree_rcu() from raw atomic context, but
the required change to permit this is quite a bit simpler.

Thanx, Paul