From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail.linuxfoundation.org ([140.211.169.12]:38540 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751633AbdITJUR (ORCPT ); Wed, 20 Sep 2017 05:20:17 -0400 Subject: Patch "srcu: Provide ordering for CPU not involved in grace period" has been added to the 4.13-stable tree To: paulmck@linux.vnet.ibm.com, gregkh@linuxfoundation.org, ldr709@gmail.com Cc: , From: Date: Wed, 20 Sep 2017 11:20:10 +0200 Message-ID: <150589921014529@kroah.com> MIME-Version: 1.0 Content-Type: text/plain; charset=ANSI_X3.4-1968 Content-Transfer-Encoding: 8bit Sender: stable-owner@vger.kernel.org List-ID: This is a note to let you know that I've just added the patch titled srcu: Provide ordering for CPU not involved in grace period to the 4.13-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: srcu-provide-ordering-for-cpu-not-involved-in-grace-period.patch and it can be found in the queue-4.13 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let know about it. >>From 35732cf9dd38b1efb0f2f22c91c61b51337d1ac3 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Wed, 5 Jul 2017 13:30:21 -0700 Subject: srcu: Provide ordering for CPU not involved in grace period From: Paul E. McKenney commit 35732cf9dd38b1efb0f2f22c91c61b51337d1ac3 upstream. Tree RCU guarantees that every online CPU has a memory barrier between any given grace period and any of that CPU's RCU read-side sections that must be ordered against that grace period. Since RCU doesn't always know where read-side critical sections are, the actual implementation guarantees order against prior and subsequent non-idle non-offline code, whether in an RCU read-side critical section or not. As a result, there does not need to be a memory barrier at the end of synchronize_rcu() and friends because the ordering internal to the grace period has ordered every CPU's post-grace-period execution against each CPU's pre-grace-period execution, again for all non-idle online CPUs. In contrast, SRCU can have non-idle online CPUs that are completely uninvolved in a given SRCU grace period, for example, a CPU that never runs any SRCU read-side critical sections and took no part in the grace-period processing. It is in theory possible for a given synchronize_srcu()'s wakeup to be delivered to a CPU that was completely uninvolved in the prior SRCU grace period, which could mean that the code following that synchronize_srcu() would end up being unordered with respect to both the grace period and any pre-existing SRCU read-side critical sections. This commit therefore adds an smp_mb() to the end of __synchronize_srcu(), which prevents this scenario from occurring. Reported-by: Lance Roy Signed-off-by: Paul E. McKenney Acked-by: Lance Roy Signed-off-by: Greg Kroah-Hartman --- kernel/rcu/srcutree.c | 9 +++++++++ 1 file changed, 9 insertions(+) --- a/kernel/rcu/srcutree.c +++ b/kernel/rcu/srcutree.c @@ -896,6 +896,15 @@ static void __synchronize_srcu(struct sr __call_srcu(sp, &rcu.head, wakeme_after_rcu, do_norm); wait_for_completion(&rcu.completion); destroy_rcu_head_on_stack(&rcu.head); + + /* + * Make sure that later code is ordered after the SRCU grace + * period. This pairs with the raw_spin_lock_irq_rcu_node() + * in srcu_invoke_callbacks(). Unlike Tree RCU, this is needed + * because the current CPU might have been totally uninvolved with + * (and thus unordered against) that grace period. + */ + smp_mb(); } /** Patches currently in stable-queue which might be from paulmck@linux.vnet.ibm.com are queue-4.13/srcu-provide-ordering-for-cpu-not-involved-in-grace-period.patch queue-4.13/smp-hotplug-handle-removal-correctly-in-cpuhp_store_callbacks.patch