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 17:36:38 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     "Paul E. McKenney" <paulmck@...nel.org>
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 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.

> > > 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.

> 							Thanx, Paul

Sebastian