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

From: Joel Fernandes <joel@joelfernandes.org>
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: "Uladzislau Rezki (Sony)" <urezki@gmail.com>,
	LKML <linux-kernel@vger.kernel.org>, RCU <rcu@vger.kernel.org>,
	linux-mm <linux-mm@kvack.org>,
	"Paul E . McKenney" <paulmck@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	"Theodore Y . Ts'o" <tytso@mit.edu>,
	Matthew Wilcox <willy@infradead.org>,
	Oleksiy Avramchenko <oleksiy.avramchenko@sonymobile.com>
Subject: Re: [PATCH 1/1] rcu/tree: Drop the lock before entering to page allocator
Date: Wed, 15 Jul 2020 19:13:33 -0400	[thread overview]
Message-ID: <CAEXW_YRoTvQfqqcM9fi+MkMxCPEaoJh4zHRM3qNYkv=-nAVuBQ@mail.gmail.com> (raw)
In-Reply-To: <20200715185628.7b4k3o5efp4gnbla@linutronix.de>

On Wed, Jul 15, 2020 at 2:56 PM Sebastian Andrzej Siewior
<bigeasy@linutronix.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