linux-kernel - Re: [PATCH] Convert struct pid count to refcount

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Date:   Thu, 28 Mar 2019 17:26:42 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Jann Horn <jannh@...gle.com>
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        Kees Cook <keescook@...omium.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Android Kernel Team <kernel-team@...roid.com>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        Michal Hocko <mhocko@...e.com>,
        "Reshetova, Elena" <elena.reshetova@...el.com>
Subject: Re: [PATCH] Convert struct pid count to refcount_t

On 03/28, Jann Horn wrote:
>
> Since we're just talking about RCU stuff now, adding Paul McKenney to
> the thread.

Since you added Paul let me add more confusion to this thread ;)

There were some concerns about the lack of barriers in put_pid(), but I can't
find that old discussion and I forgot the result of that discussion...

Paul, could you confirm that this code

	CPU_0		CPU_1

	X = 1;		if (READ_ONCE(Y))
	mb();			X = 2;
	Y = 1;		BUG_ON(X != 2);


is correct? I think it is, control dependency pairs with mb(), right?

If not, then put_pid() needs atomic_read_acquire() as it was proposed in that
discussion.

Oleg.