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Date:   Thu, 16 Jul 2020 08:20:27 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc:     Uladzislau Rezki <urezki@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>, RCU <rcu@vger.kernel.org>,
        linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
        "Theodore Y . Ts'o" <tytso@mit.edu>,
        Matthew Wilcox <willy@infradead.org>,
        Joel Fernandes <joel@joelfernandes.org>,
        Oleksiy Avramchenko <oleksiy.avramchenko@sonymobile.com>
Subject: Re: [PATCH 1/1] rcu/tree: Drop the lock before entering to page
 allocator
Message-ID: <20200716152027.GQ9247@paulmck-ThinkPad-P72>
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On Thu, Jul 16, 2020 at 04:14:21PM +0200, Sebastian Andrzej Siewior wrote:
> On 2020-07-15 15:14:49 [-0700], Paul E. McKenney wrote:
> > 
> > My concern is that some critical bug will show up at some point
> > that requires double-argument kfree_rcu() be invoked while holding
> > a raw spinlock.  (Single-argument kfree_rcu() must sometimes invoke
> > synchronize_rcu(), so it can never be invoked in any state forbidding
> > invoking schedule().)
> 
> So you are saying as of today we are good but in near future the
> following
>    synchronize_rcu() -> kfree_rcu()
> 
> may be needed?

You lost me on this one.  I am instead concerned that something like this
might be needed on short notice:

	raw_spin_lock(&some_lock);
	kfree_rcu(some_pointer, some_field_offset);

In contrast, single-argument kfree_rcu() cannot be invoked from any
environment where synchronize_rcu() cannot be invoked.

> > Yes, dropping to a plain spinlock would be simple in the here and now,
> > but experience indicates that it is only a matter of time, and that when
> > that time comes it will come as an emergency.
> 
> Hmmm.

I point out the call_rcu() experience.

> > One approach would be to replace the "IS_ENABLED(CONFIG_PREEMPT_RT)"
> > with some sort of check for being in a context where spinlock acquisition
> > is not legal.  What could be done along those lines?
> 
> I would rethink the whole concept how this is implemented now and give
> it another try. The code does not look pretty and is looking
> complicated. The RT covering of this part then just added a simple
> return because nothing else seemed to be possible. This patch here
> looks like another duct tape attempt to avoid a warning.

In addition to the possibility of invocation from BH?

							Thanx, Paul