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

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Message-ID: <877dpqprc2.fsf@x220.int.ebiederm.org>
Date:   Wed, 09 Dec 2020 16:06:21 -0600
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Al Viro <viro@...iv.linux.org.uk>, 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

Matthew Wilcox <willy@...radead.org> writes:

> 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.

Thanks.  I was remembering there was a special case like and I had
forgotten what the rule was.

Eric