linux-kernel - Re: [PATCH 1/1] rcu/tree: Drop the lock before entering to page allocator

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Date:   Wed, 15 Jul 2020 19:13:33 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     "Uladzislau Rezki (Sony)" <urezki@...il.com>,
        LKML <linux-kernel@...r.kernel.org>, RCU <rcu@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Theodore Y . Ts'o" <tytso@....edu>,
        Matthew Wilcox <willy@...radead.org>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>
Subject: Re: [PATCH 1/1] rcu/tree: Drop the lock before entering to page allocator

On Wed, Jul 15, 2020 at 2:56 PM Sebastian Andrzej Siewior
<bigeasy@...utronix.de> wrote:
>
> On 2020-07-15 20:35:37 [+0200], Uladzislau Rezki (Sony) wrote:
> > @@ -3306,6 +3307,9 @@ kvfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp, void *ptr)
> >                       if (IS_ENABLED(CONFIG_PREEMPT_RT))
> >                               return false;
> >
> > +                     preempt_disable();
> > +                     krc_this_cpu_unlock(*krcp, *flags);
>
> Now you enter memory allocator with disabled preemption. This isn't any
> better but we don't have a warning for this yet.
> What happened to the part where I asked for a spinlock_t?

Ulad,
Wouldn't the replacing of preempt_disable() with migrate_disable()
above resolve Sebastian's issue?

Or which scenario breaks?

thanks,

 - Joel


>
> > +
> >                       /*
> >                        * NOTE: For one argument of kvfree_rcu() we can
> >                        * drop the lock and get the page in sleepable
>
> Sebastian