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 12:56:01 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mel Gorman <mgorman@...hsingularity.net>
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 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.