linux-kernel - Re: [patch 1/2] mm: protect local lock sections with rcu_read

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Date:   Tue, 22 Feb 2022 17:16:53 +0100
From:   Nicolas Saenz Julienne <nsaenzju@...hat.com>
To:     Marcelo Tosatti <mtosatti@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Minchan Kim <minchan@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Juri Lelli <juril@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        "Paul E. McKenney" <paulmck@...nel.org>
Subject: Re: [patch 1/2] mm: protect local lock sections with rcu_read_lock
 (on RT)

On Tue, 2022-02-22 at 12:51 -0300, Marcelo Tosatti wrote:
> On Tue, Feb 22, 2022 at 04:21:26PM +0100, Nicolas Saenz Julienne wrote:
> > On Tue, 2022-02-22 at 11:47 -0300, Marcelo Tosatti wrote:
> > > For the per-CPU LRU page vectors, augment the local lock protected
> > > code sections with rcu_read_lock.
> > > 
> > > This makes it possible to replace the queueing of work items on all 
> > > CPUs by synchronize_rcu (which is necessary to run FIFO:1 applications
> > > uninterrupted on isolated CPUs).
> > 
> > I don't think this is needed. In RT local_locks use a spinlock. See
> > kernel/locking/spinlock_rt.c:
> > 
> > "The RT [spinlock] substitutions explicitly disable migration and take
> > rcu_read_lock() across the lock held section."
> 
> Nice! Then the migrate_disable from __local_lock and friends seems unnecessary as
> well
>
> #define __local_lock(__lock)                                    \
>         do {                                                    \
>                 migrate_disable();                              \
>                 spin_lock(this_cpu_ptr((__lock)));              \
>         } while (0)
> 

It's needed as you might migrate between:

	cpu1_lock = this_cpu_ptr(__lock);
	// migrate here to cpu2
	spin_lock(cpu1_lock);
	// unprotected write into cpu2 lists

Regards,

-- 
Nicolás Sáenz