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

[Michal Hocko]

On Thu 13-08-20 09:29:04, Paul E. McKenney wrote:
> 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.

Which would implemente a completely new level atomic allocation for all
preemption models

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

this would have to be RT specific to not change the semantic for
existing users. In other words make NOWAIT semantic working for
RT atomic contexts.

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

and this would have to go along with 3 to remove false positives on !RT.

-- 
Michal Hocko
SUSE Labs