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

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Date:   Thu, 8 Apr 2021 18:48:04 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Linux-MM <linux-mm@...ck.org>,
        Linux-RT-Users <linux-rt-users@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Chuck Lever <chuck.lever@...cle.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Michal Hocko <mhocko@...nel.org>,
        Oscar Salvador <osalvador@...e.de>
Subject: Re: [PATCH 0/11 v2] Use local_lock for pcp protection and reduce
 stat overhead

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