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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
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