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

From: Paul E. McKenney
Date: Thu Aug 13 2020 - 12:29:07 EST

On Thu, Aug 13, 2020 at 06:13:57PM +0200, Michal Hocko wrote:
> On Thu 13-08-20 09:04:42, Paul E. McKenney wrote:
> > On Thu, Aug 13, 2020 at 05:54:12PM +0200, Michal Hocko wrote:
> [...]
> > > If the whole bailout is guarded by CONFIG_PREEMPT_RT specific atomicity
> > > check then there is no functional problem - GFP_RT_SAFE would still be
> > > GFP_NOWAIT so functional wise the allocator will still do the right
> > > thing.
> >
> > Perhaps it was just me getting confused, early hour Pacific Time and
> > whatever other excuses might apply. But I thought that you still had
> > an objection to GFP_RT_SAFE based on changes in allocator semantics for
> > other users.
> There is still that problem with lockdep complaining about raw->regular
> spinlock on !PREEMPT_RT that would need to get resolved somehow. Thomas
> is not really keen on adding some lockdep annotation mechanism and
> unfortunatelly I do not have a different idea how to get rid of those.

OK. So the current situation requires a choice between these these
alternatives, each of which has shortcomings that have been mentioned
earlier in this thread:

1. Prohibit invoking allocators from raw atomic context, such
as when holding a raw spinlock.

2. Adding a GFP_ flag.

3. Reusing existing GFP_ flags/values/whatever to communicate
the raw-context information that was to be communicated by
the new GFP_ flag.

4. Making lockdep forgive acquiring spinlocks while holding
raw spinlocks, but only in CONFIG_PREEMPT_NONE=y kernels.

Am I missing anything?

Thanx, Paul