linux-kernel - Re: [PATCH v8 07/11] proc: flush task dcache entries from all procfs instances

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Message-ID: <20200212204124.GR23230@ZenIV.linux.org.uk>
Date:   Wed, 12 Feb 2020 20:41:24 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Linux API <linux-api@...r.kernel.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        Linux Security Module <linux-security-module@...r.kernel.org>,
        Akinobu Mita <akinobu.mita@...il.com>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Daniel Micay <danielmicay@...il.com>,
        Djalal Harouni <tixxdz@...il.com>,
        "Dmitry V . Levin" <ldv@...linux.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        "J . Bruce Fields" <bfields@...ldses.org>,
        Jeff Layton <jlayton@...chiereds.net>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Solar Designer <solar@...nwall.com>
Subject: Re: [PATCH v8 07/11] proc: flush task dcache entries from all procfs
 instances

On Wed, Feb 12, 2020 at 08:38:33PM +0000, Al Viro wrote:
> On Wed, Feb 12, 2020 at 12:35:04PM -0800, Linus Torvalds wrote:
> > On Wed, Feb 12, 2020 at 12:03 PM Al Viro <viro@...iv.linux.org.uk> wrote:
> > >
> > > What's to prevent racing with fs shutdown while you are doing the second part?
> > 
> > I was thinking that only the proc_flush_task() code would do this.
> > 
> > And that holds a ref to the vfsmount through upid->ns.
> > 
> > So I wasn't suggesting doing this in general - just splitting up the
> > implementation of d_invalidate() so that proc_flush_task_mnt() could
> > delay the complex part to after having traversed the RCU-protected
> > list.
> > 
> > But hey - I missed this part of the problem originally, so maybe I'm
> > just missing something else this time. Wouldn't be the first time.
> 
> Wait, I thought the whole point of that had been to allow multiple
> procfs instances for the same userns?  Confused...

s/userns/pidns/, sorry