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

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Message-ID: <20201209225828.GR3579531@ZenIV.linux.org.uk>
Date:   Wed, 9 Dec 2020 22:58:28 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.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 07:49:38PM +0000, Matthew Wilcox wrote:
> On Wed, Dec 09, 2020 at 12:04:38PM -0600, Eric W. Biederman wrote:
> > @@ -397,8 +397,9 @@ static struct fdtable *close_files(struct files_struct * files)
> >  		set = fdt->open_fds[j++];
> >  		while (set) {
> >  			if (set & 1) {
> > -				struct file * file = xchg(&fdt->fd[i], NULL);
> > +				struct file * file = fdt->fd[i];
> >  				if (file) {
> > +					rcu_assign_pointer(fdt->fd[i], NULL);
> 
> 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...