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Message-ID: <CAHk-=wgwmu4jpmOqW0+Lz0dcem1Fub=ThLHvmLobf_WqCq7bwg@mail.gmail.com>
Date: Wed, 12 Feb 2020 11:49:58 -0800
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: "Eric W. Biederman" <ebiederm@...ssion.com>
Cc: 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>, Alexander Viro <viro@...iv.linux.org.uk>,
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 11:18 AM Eric W. Biederman
<ebiederm@...ssion.com> wrote:
>
> > So it's just fs_info that needs to be rcu-delayed because it contains
> > that list. Or is there something else?
>
> The fundamental dcache thing we are playing with is:
>
> dentry = d_hash_and_lookup(proc_root, &name);
> if (dentry) {
> d_invalidate(dentry);
> dput(dentry);
> }
Ahh. And we can't do that part under the RCU read lock. So it's not
the freeing, it's the list traversal itself.
Fair enough.
Hmm.
I wonder if we could split up d_invalidate(). It already ends up being
two phases: first the unhashing under the d_lock, and then the
recursive shrinking of parents and children.
The recursive shrinking of the parent isn't actually interesting for
the proc shrinking case: we just looked up one child, after all. So we
only care about the d_walk of the children.
So if we only did the first part under the RCU lock, and just
collected the dentries (can we perhaps then re-use the hash list to
collect them to another list?) and then did the child d_walk
afterwards?
Linus
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