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
Date: Thu, 16 Jul 2020 08:20:27 -0700 [thread overview]
Message-ID: <20200716152027.GQ9247@paulmck-ThinkPad-P72> (raw)
In-Reply-To: <20200716141421.fzwf4tedr6rixd6d@linutronix.de>
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
next prev parent reply other threads:[~2020-07-16 15:20 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-15 18:35 [PATCH 1/1] rcu/tree: Drop the lock before entering to page allocator Uladzislau Rezki (Sony)
2020-07-15 18:56 ` Sebastian Andrzej Siewior
2020-07-15 19:02 ` Uladzislau Rezki
2020-07-15 19:32 ` Sebastian Andrzej Siewior
2020-07-15 19:36 ` Uladzislau Rezki
2020-07-15 22:14 ` Paul E. McKenney
2020-07-16 14:14 ` Sebastian Andrzej Siewior
2020-07-16 15:20 ` Paul E. McKenney [this message]
2020-07-16 15:36 ` Sebastian Andrzej Siewior
2020-07-16 16:36 ` Paul E. McKenney
2020-07-15 23:13 ` Joel Fernandes
2020-07-16 9:19 ` Uladzislau Rezki
2020-07-16 13:36 ` Joel Fernandes
2020-07-16 14:37 ` Uladzislau Rezki
2020-07-16 18:27 ` Joel Fernandes
2020-07-16 19:03 ` Uladzislau Rezki
2020-07-16 14:25 ` Sebastian Andrzej Siewior
2020-07-16 14:47 ` Uladzislau Rezki
2020-07-16 15:04 ` Sebastian Andrzej Siewior
2020-07-16 15:34 ` Uladzislau Rezki
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