linux-kernel - Re: [PATCH 1/1] rcu/tree: Drop the lock before entering to page allocator

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Message-ID: <20200715221449.GJ9247@paulmck-ThinkPad-P72>
Date:   Wed, 15 Jul 2020 15:14:49 -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 Wed, Jul 15, 2020 at 09:32:50PM +0200, Sebastian Andrzej Siewior wrote:
> On 2020-07-15 21:02:43 [+0200], Uladzislau Rezki wrote:
> > 
> > <snip>
> >     spin_lock();
> >     __get_free_page(GFP_NOWAIT | __GFP_NOWARN);
> >     spin_unlock();
> > <snip>
> > 
> > Also, please note we do it for regular kernel.
> 
> ach right okay then.
> 
> > >
> > > What happened to the part where I asked for a spinlock_t?
> > > 
> > What do you mean?
> 
> Please drop that raw_spinlock_t for the kfree_rcu() based locking and
> use just a plain spinlock_t for the locking. Then you can keep the same
> code flow for RT and !RT without any special cases and everything.

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().)

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.

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?

							Thanx, Paul