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

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Date:   Thu, 16 Jul 2020 11:19:13 +0200
From:   Uladzislau Rezki <urezki@...il.com>
To:     Joel Fernandes <joel@...lfernandes.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        "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 07:13:33PM -0400, Joel Fernandes wrote:
> 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?
>
This for regular kernel only. That means that migrate_disable() is
equal to preempt_disable(). So, no difference.

> 
> Or which scenario breaks?
> 
Proposed patch fixes Sebastian's finding about CONFIG_PROVE_RAW_LOCK_NESTING
kernel option, that checks nesting rules and forbids raw_spinlock versus
spinlock mixing.

Sebastian, could you please confirm that if that patch that is in
question fixes it?

It would be appreciated!

--
Vlad Rezki