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

[Paul E. McKenney]

On Tue, Aug 11, 2020 at 09:02:40AM -0700, Paul E. McKenney wrote:
> On Tue, Aug 11, 2020 at 05:43:16PM +0200, Thomas Gleixner wrote:
> > "Paul E. McKenney" <paulmck@xxxxxxxxxx> 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.

I should hasten to add that from what I can see right now, the required
change allows telling the memory allocator bail out instead of acquiring
a non-raw spinlock.  I am absolutely not advocating converting the
allocator's spinlocks to raw spinlocks.

							Thanx, Paul