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 09:36:51 -0700 [thread overview]
Message-ID: <20200716163651.GT9247@paulmck-ThinkPad-P72> (raw)
In-Reply-To: <20200716153638.gfh6dzp2h35ygfaa@linutronix.de>
On Thu, Jul 16, 2020 at 05:36:38PM +0200, Sebastian Andrzej Siewior wrote:
> On 2020-07-16 08:20:27 [-0700], Paul E. McKenney wrote:
> > 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.
>
> I see. We don't have any kfree() in that context as far as I remember.
> We had a few cases in "resize" where you allocate memory, copy content
> and free old memory while under the lock but they are gone.
True, but we also didn't have any calls to call_rcu() prior to the call
to rcu_init() until suddenly we did. (Yeah, I could have put my foot
down and prohibited that practice, but the workarounds were quite a bit
more complicated than just making call_rcu() work during very early boot.)
And last I checked, there really were calls to call_rcu() under raw
spinlocks, so the potential or calls to double-argument kfree_rcu()
clearly exists and is very real.
> > > > 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?
>
> Invocation from BH should be possible because network would probably be
> the first user. I don't remember anything wrong with BH if I remember
> correctly.
OK, that is reassuring. Here is hoping!
Thanx, Paul
next prev parent reply other threads:[~2020-07-16 16:36 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
2020-07-16 15:36 ` Sebastian Andrzej Siewior
2020-07-16 16:36 ` Paul E. McKenney [this message]
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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20200716163651.GT9247@paulmck-ThinkPad-P72 \
--to=paulmck@kernel.org \
--cc=akpm@linux-foundation.org \
--cc=bigeasy@linutronix.de \
--cc=joel@joelfernandes.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=oleksiy.avramchenko@sonymobile.com \
--cc=rcu@vger.kernel.org \
--cc=tytso@mit.edu \
--cc=urezki@gmail.com \
--cc=willy@infradead.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).