linux-kernel - Re: [PATCH] files: rcu free files

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Message-ID: <20201210003922.GL2657@paulmck-ThinkPad-P72>
Date:   Wed, 9 Dec 2020 16:39:22 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Oleg Nesterov <oleg@...hat.com>, Jann Horn <jann@...jh.net>
Subject: Re: [PATCH] files: rcu free files_struct

On Wed, Dec 09, 2020 at 03:29:09PM -0800, Linus Torvalds wrote:
> On Wed, Dec 9, 2020 at 3:07 PM Matthew Wilcox <willy@...radead.org> wrote:
> >
> >  #. It is not necessary to use rcu_assign_pointer() when creating
> >     linked structures that are to be published via a single external
> > -   pointer. The RCU_INIT_POINTER() macro is provided for this task
> > -   and also for assigning ``NULL`` pointers at runtime.
> > +   pointer. The RCU_INIT_POINTER() macro is provided for this task.
> > +   It used to be more efficient to use RCU_INIT_POINTER() to store a
> > +   ``NULL`` pointer, but rcu_assign_pointer() now optimises for a constant
> > +   ``NULL`` pointer itself.
> 
> I would just remove the historical note about "It used to be more
> efficient ..". It's not really helpful.
> 
> If somebody wants to dig into the history, it's there in git.
> 
> Maybe the note could be part of the commit message for the comment
> update, explaining that it used to be more efficient but no longer is.
> Because commit messages are about the explanation for the change.
> 
> But I don't feel it makes any sense in the source code.

Like this, then?

							Thanx, Paul

 #. It is not necessary to use rcu_assign_pointer() when creating
    linked structures that are to be published via a single external
-   pointer. The RCU_INIT_POINTER() macro is provided for this task
-   and also for assigning ``NULL`` pointers at runtime.
+   pointer. The RCU_INIT_POINTER() macro is provided for this task.