linux-kernel - Re: [PATCH v2 1/2] pidfd: add P

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Message-ID: <20190727164932.GR1131@ZenIV.linux.org.uk>
Date:   Sat, 27 Jul 2019 17:49:32 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Christian Brauner <christian@...uner.io>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Oleg Nesterov <oleg@...hat.com>, Arnd Bergmann <arnd@...db.de>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Kees Cook <keescook@...omium.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tejun Heo <tj@...nel.org>, David Howells <dhowells@...hat.com>,
        Jann Horn <jannh@...gle.com>,
        Andrew Lutomirski <luto@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Aleksa Sarai <cyphar@...har.com>,
        Android Kernel Team <kernel-team@...roid.com>
Subject: Re: [PATCH v2 1/2] pidfd: add P_PIDFD to waitid()

On Sat, Jul 27, 2019 at 09:28:40AM -0700, Linus Torvalds wrote:

> is the stupid and straightforward thing, but if you want to be
> *clever* you can actually avoid getting a reference to the 'struct
> file *" entirely, and do the fd->pid lookup under rcu_read_lock()
> instead. It's slightly more complex, but it avoids the fdget/fdput
> reference count games entirely.

Yecchhh...  Please, don't do the last part - at least not unless
we really see that in profiles.