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