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

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Message-ID: <CAHk-=wiUMHBHmmDS3_Xqh1wfGFyd_rdDmpZzk0cODoj1i7_VOA@mail.gmail.com>
Date:   Wed, 9 Dec 2020 11:13:38 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        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 10:05 AM Eric W. Biederman <ebiederm@...ssion.com> wrote:
>
> -                               struct file * file = xchg(&fdt->fd[i], NULL);
> +                               struct file * file = fdt->fd[i];
>                                 if (file) {
> +                                       rcu_assign_pointer(fdt->fd[i], NULL);

This makes me nervous. Why did we use to do that xchg() there? That
has atomicity guarantees that now are gone.

Now, this whole thing should be called for just the last ref of the fd
table, so presumably that atomicity was never needed in the first
place. But the fact that we did that very expensive xchg() then makes
me go "there's some reason for it".

Is this xchg() just bogus historical leftover? It kind of looks that
way. But maybe that change should be done separately?

              Linus