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.
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