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

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Message-ID: <CAHk-=wi7MDO7hSK9-7pbfuwb0HOkMQF1fXyidxR=sqrFG-ZQJg@mail.gmail.com>
Date:   Wed, 9 Dec 2020 15:01:36 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Matthew Wilcox <willy@...radead.org>,
        "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 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().

           Linus