[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <CAHk-=wgmn9Qds0VznyphouSZW6e42GWDT5H1dpZg8pyGDGN+=w@mail.gmail.com>
Date: Wed, 12 Feb 2020 16:48:14 -0800
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: "Eric W. Biederman" <ebiederm@...ssion.com>
Cc: Al Viro <viro@...iv.linux.org.uk>, 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 1:48 PM Eric W. Biederman <ebiederm@...ssion.com> wrote:
>
> The good news is proc_flush_task isn't exactly called from process exit.
> proc_flush_task is called during zombie clean up. AKA release_task.
Yeah, that at least avoids some of the nasty locking while dying debug problems.
But the one I was more worried about was actually the lock contention
issue with lots of processes. The lock is basically a single global
lock in many situations - yes, it's technically per-ns, but in a lot
of cases you really only have one namespace anyway.
And we've had problems with global locks in this area before, notably
the one you call out:
> Further after proc_flush_task does it's thing the code goes
> and does "write_lock_irq(&task_list_lock);"
Yeah, so it's not introducing a new issue, but it is potentially
making something we already know is bad even worse.
> What would be downside of having a mutex for a list of proc superblocks?
> A mutex that is taken for both reading and writing the list.
That's what the original patch actually was, and I was hoping we could
avoid that thing.
An rwsem would be possibly better, since most cases by far are likely
about reading.
And yes, I'm very aware of the task_list_lock, but it's literally why
I don't want to make a new one.
I'm _hoping_ we can some day come up with something better than task_list_lock.
Linus
Powered by blists - more mailing lists
Confused about mailing lists and their use?
Read about mailing lists on Wikipedia
and check out these
guidelines on proper formatting of your messages.
