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

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Message-ID: <20201209230755.GV7338@casper.infradead.org>
Date:   Wed, 9 Dec 2020 23:07:55 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     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>,
        "Paul E. McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH] files: rcu free files_struct

On Wed, Dec 09, 2020 at 03:01:36PM -0800, Linus Torvalds wrote:
> On Wed, Dec 9, 2020 at 2:58 PM Al Viro <viro@...iv.linux.org.uk> wrote:
> >
> > On Wed, Dec 09, 2020 at 07:49:38PM +0000, Matthew Wilcox wrote:
> > >
> > > Assuming this is safe, you can use RCU_INIT_POINTER() here because you're
> > > storing NULL, so you don't need the wmb() before storing the pointer.
> >
> > fs/file.c:pick_file() would make more interesting target for the same treatment...
> 
> Actually, don't.
> 
> rcu_assign_pointer() itself already does the optimization for the case
> of a constant NULL pointer assignment.
> 
> So there's no need to manually change things to RCU_INIT_POINTER().

I missed that, and the documentation wasn't updated by
3a37f7275cda5ad25c1fe9be8f20c76c60d175fa.

Paul, how about this?

+++ b/Documentation/RCU/Design/Requirements/Requirements.rst
@@ -1668,8 +1668,10 @@ against mishaps and misuse:
    this purpose.
 #. 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.
 
 This not a hard-and-fast list: RCU's diagnostic capabilities will
 continue to be guided by the number and type of usage bugs found in