[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <20200716163651.GT9247@paulmck-ThinkPad-P72>
Date: Thu, 16 Jul 2020 09:36:51 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: Uladzislau Rezki <urezki@...il.com>,
LKML <linux-kernel@...r.kernel.org>, RCU <rcu@...r.kernel.org>,
linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
"Theodore Y . Ts'o" <tytso@....edu>,
Matthew Wilcox <willy@...radead.org>,
Joel Fernandes <joel@...lfernandes.org>,
Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>
Subject: Re: [PATCH 1/1] rcu/tree: Drop the lock before entering to page
allocator
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
Powered by blists - more mailing lists