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 16:14:21 +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 2020-07-15 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?

> 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.

> 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.

> 							Thanx, Paul

Sebastian