Re: [PATCH 0/11 v2] Use local_lock for pcp protection and reduce stat overhead

[Mel Gorman <mgorman@techsingularity.net>]

On Thu, Apr 08, 2021 at 12:56:01PM +0200, Peter Zijlstra wrote:
> On Wed, Apr 07, 2021 at 09:24:12PM +0100, Mel Gorman wrote:
> > Why local_lock? PREEMPT_RT considers the following sequence to be unsafe
> > as documented in Documentation/locking/locktypes.rst
> > 
> >    local_irq_disable();
> >    raw_spin_lock(&lock);
> 
> Almost, the above is actually OK on RT. The problematic one is:
> 
> 	local_irq_disable();
> 	spin_lock(&lock);
> 
> That doesn't work on RT since spin_lock() turns into a PI-mutex which
> then obviously explodes if it tries to block with IRQs disabled.
> 
> And it so happens, that's exactly the one at hand.

Ok, I completely messed up the leader because it was local_irq_disable()
+ spin_lock() that I was worried about. Once the series is complete,
it is replated with

  local_lock_irq(&lock_lock)
  spin_lock(&lock);

According to Documentation/locking/locktypes.rst, that should be safe.
I'll rephrase the justification.

-- 
Mel Gorman
SUSE Labs