From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail.linuxfoundation.org ([140.211.169.12]:51312 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753144AbdKJNzy (ORCPT ); Fri, 10 Nov 2017 08:55:54 -0500 Subject: Patch "MIPS: SMP: Fix deadlock & online race" has been added to the 4.4-stable tree To: matt.redfearn@imgtec.com, gregkh@linuxfoundation.org, jhogan@kernel.org, matija.glavinic-pecotic.ext@nokia.com Cc: , From: Date: Fri, 10 Nov 2017 14:55:36 +0100 Message-ID: <151032213621200@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 MIPS: SMP: Fix deadlock & online race to the 4.4-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: mips-smp-fix-deadlock-online-race.patch and it can be found in the queue-4.4 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let know about it. >>From 9e8c399a88f0b87e41a894911475ed2a8f8dff9e Mon Sep 17 00:00:00 2001 From: Matt Redfearn Date: Wed, 27 Sep 2017 10:13:25 +0100 Subject: MIPS: SMP: Fix deadlock & online race From: Matt Redfearn commit 9e8c399a88f0b87e41a894911475ed2a8f8dff9e upstream. Commit 6f542ebeaee0 ("MIPS: Fix race on setting and getting cpu_online_mask") effectively reverted commit 8f46cca1e6c06 ("MIPS: SMP: Fix possibility of deadlock when bringing CPUs online") and thus has reinstated the possibility of deadlock. The commit was based on testing of kernel v4.4, where the CPU hotplug core code issued a BUG() if the starting CPU is not marked online when the boot CPU returns from __cpu_up. The commit fixes this race (in v4.4), but re-introduces the deadlock situation. As noted in the commit message, upstream differs in this area. Commit 8df3e07e7f21f ("cpu/hotplug: Let upcoming cpu bring itself fully up") adds a completion event in the CPU hotplug core code, making this race impossible. However, people were unhappy with relying on the core code to do the right thing. To address the issues both commits were trying to fix, add a second completion event in the MIPS smp hotplug path. It removes the possibility of a race, since the MIPS smp hotplug code now synchronises both the boot and secondary CPUs before they return to the hotplug core code. It also addresses the deadlock by ensuring that the secondary CPU is not marked online before it's counters are synchronised. This fix should also be backported to fix the race condition introduced by the backport of commit 8f46cca1e6c06 ("MIPS: SMP: Fix possibility of deadlock when bringing CPUs online"), through really that race only existed before commit 8df3e07e7f21f ("cpu/hotplug: Let upcoming cpu bring itself fully up"). Signed-off-by: Matt Redfearn Fixes: 6f542ebeaee0 ("MIPS: Fix race on setting and getting cpu_online_mask") CC: Matija Glavinic Pecotic Patchwork: https://patchwork.linux-mips.org/patch/17376/ Signed-off-by: James Hogan [jhogan@kernel.org: Backported 4.1..4.9] Signed-off-by: Greg Kroah-Hartman --- arch/mips/kernel/smp.c | 22 ++++++++++++++++------ 1 file changed, 16 insertions(+), 6 deletions(-) --- a/arch/mips/kernel/smp.c +++ b/arch/mips/kernel/smp.c @@ -64,6 +64,7 @@ EXPORT_SYMBOL(cpu_sibling_map); cpumask_t cpu_core_map[NR_CPUS] __read_mostly; EXPORT_SYMBOL(cpu_core_map); +static DECLARE_COMPLETION(cpu_starting); static DECLARE_COMPLETION(cpu_running); /* @@ -176,6 +177,12 @@ asmlinkage void start_secondary(void) cpumask_set_cpu(cpu, &cpu_coherent_mask); notify_cpu_starting(cpu); + /* Notify boot CPU that we're starting & ready to sync counters */ + complete(&cpu_starting); + + synchronise_count_slave(cpu); + + /* The CPU is running and counters synchronised, now mark it online */ set_cpu_online(cpu, true); set_cpu_sibling_map(cpu); @@ -183,8 +190,11 @@ asmlinkage void start_secondary(void) calculate_cpu_foreign_map(); + /* + * Notify boot CPU that we're up & online and it can safely return + * from __cpu_up + */ complete(&cpu_running); - synchronise_count_slave(cpu); /* * irq will be enabled in ->smp_finish(), enabling it too early @@ -250,17 +260,17 @@ int __cpu_up(unsigned int cpu, struct ta { mp_ops->boot_secondary(cpu, tidle); - /* - * We must check for timeout here, as the CPU will not be marked - * online until the counters are synchronised. - */ - if (!wait_for_completion_timeout(&cpu_running, + /* Wait for CPU to start and be ready to sync counters */ + if (!wait_for_completion_timeout(&cpu_starting, msecs_to_jiffies(1000))) { pr_crit("CPU%u: failed to start\n", cpu); return -EIO; } synchronise_count_master(cpu); + + /* Wait for CPU to finish startup & mark itself online before return */ + wait_for_completion(&cpu_running); return 0; } Patches currently in stable-queue which might be from matt.redfearn@imgtec.com are queue-4.4/mips-smp-fix-deadlock-online-race.patch queue-4.4/mips-smp-use-a-completion-event-to-signal-cpu-up.patch