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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Wed, 12 Feb 2020 16:48:14 -0800
Message-ID: <CAHk-=wgmn9Qds0VznyphouSZW6e42GWDT5H1dpZg8pyGDGN+=w@mail.gmail.com>
Subject: Re: [PATCH v8 07/11] proc: flush task dcache entries from all procfs instances
To:     "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     Al Viro <viro@zeniv.linux.org.uk>,
        LKML <linux-kernel@vger.kernel.org>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        Linux API <linux-api@vger.kernel.org>,
        Linux FS Devel <linux-fsdevel@vger.kernel.org>,
        Linux Security Module <linux-security-module@vger.kernel.org>,
        Akinobu Mita <akinobu.mita@gmail.com>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andy Lutomirski <luto@kernel.org>,
        Daniel Micay <danielmicay@gmail.com>,
        Djalal Harouni <tixxdz@gmail.com>,
        "Dmitry V . Levin" <ldv@altlinux.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Ingo Molnar <mingo@kernel.org>,
        "J . Bruce Fields" <bfields@fieldses.org>,
        Jeff Layton <jlayton@poochiereds.net>,
        Jonathan Corbet <corbet@lwn.net>,
        Kees Cook <keescook@chromium.org>,
        Oleg Nesterov <oleg@redhat.com>,
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On Wed, Feb 12, 2020 at 1:48 PM Eric W. Biederman <ebiederm@xmission.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