Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752561AbdGaPqQ (ORCPT ); Mon, 31 Jul 2017 11:46:16 -0400 Received: from merlin.infradead.org ([205.233.59.134]:43214 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752063AbdGaPqN (ORCPT ); Mon, 31 Jul 2017 11:46:13 -0400 Date: Mon, 31 Jul 2017 17:45:56 +0200 From: Peter Zijlstra To: Paolo Bonzini Cc: torvalds@linux-foundation.org, linux-kernel@vger.kernel.org, tglx@linutronix.de, mingo@kernel.org, dvyukov@google.com, hpa@zytor.com, linux-tip-commits@vger.kernel.org, Steven Rostedt , Dima Zavin Subject: Re: [tip:locking/urgent] locking/static_key: Fix concurrent static_key_slow_inc() Message-ID: <20170731154556.w7c3vfdgottrcwi5@hirez.programming.kicks-ass.net> References: <1466527937-69798-1-git-send-email-pbonzini@redhat.com> <20170731093612.7v23kxkk47i56io6@hirez.programming.kicks-ass.net> <671056c7-5fc7-60c0-4035-b11d43d95bf1@redhat.com> <20170731133349.gphnv5pqkn2xibwf@hirez.programming.kicks-ass.net> <645f28cb-56c8-7298-568f-357a1b8929b9@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <645f28cb-56c8-7298-568f-357a1b8929b9@redhat.com> User-Agent: NeoMutt/20170609 (1.8.3) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2407 Lines: 57 On Mon, Jul 31, 2017 at 05:35:40PM +0200, Paolo Bonzini wrote: > On 31/07/2017 15:33, Peter Zijlstra wrote: > > On Mon, Jul 31, 2017 at 03:04:06PM +0200, Paolo Bonzini wrote: > >> - the smp_mb (though it could be a smp_wmb or atomic_set_release) > >> initially triggered my paranoia indeed. But even then, I can't see why > >> you would need it because there's nothing it pairs with. > > > > So this one would pair with the mb implied by the cmpxchg loop for > > inc-if-positive. The ordering being that if we see a positive v, we must > > then also see all the text modifications. > > > > So if jump_label_update() were to not fully serialize things, it would > > be possible for the v=1 store to appear before the last text changes. > > And in that case we'd allow the fast path to complete > > static_key_slow_inc() before it was in fact done with changing all text. > > > > Now, I suspect (but did not audit) that anything that changes text must > > in fact serialize world, but I wasn't sure. > > I see. Then yes, I agree a smp_wmb would be nicer here. I'll make it atomic_set_release() and a comment. > >> Rather, it's *any use of key->enabled outside jump_label_lock* > >> (meaning: any use of static_key_enabled and static_key_count outside > >> the core jump_label.c code) that should be handled with care. > > > >> - net/ipv4/udp.c and net/ipv6/udp.c want to implement a "once-only" > >> increment of the static key. It's racy and maybe we should provide a > >> new API static_key_enable_forever: > >> > >> void static_key_enable_forever(struct static_key *key) > >> { > >> STATIC_KEY_CHECK_USE(); > >> if (atomic_read(&key->enabled) > 0) > >> return; > >> > >> cpus_read_lock(); > >> jump_label_lock(); > >> if (atomic_read(&key->enabled) == 0) { > >> atomic_set(&key->enabled, -1); > >> jump_label_update(key); > >> atomic_set(&key->enabled, 1); > >> } > >> jump_label_unlock(); > >> cpus_read_unlock(); > >> } > >> EXPORT_SYMBOL_GPL(static_key_enable_forever); > >> > >> I can prepare a patch if you agree. > > > > Isn't that what we have static_key_enable() for? Which btw also uses > > static_key_count() outside of the mutex. > > Yes, they should be fixed and net/ can then use static_key_enable. Right, let me try and fix _enable().