From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id D940BC001B0 for ; Thu, 10 Aug 2023 17:22:01 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233668AbjHJRWA (ORCPT ); Thu, 10 Aug 2023 13:22:00 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:32768 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232925AbjHJRV4 (ORCPT ); Thu, 10 Aug 2023 13:21:56 -0400 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id 7E4342702 for ; Thu, 10 Aug 2023 10:21:55 -0700 (PDT) Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 9CCADD75; Thu, 10 Aug 2023 10:22:37 -0700 (PDT) Received: from [10.1.196.40] (e121345-lin.cambridge.arm.com [10.1.196.40]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 6A1223F6C4; Thu, 10 Aug 2023 10:21:54 -0700 (PDT) Message-ID: Date: Thu, 10 Aug 2023 18:21:47 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux aarch64; rv:102.0) Gecko/20100101 Thunderbird/102.13.0 Subject: Re: [PATCH v5] perf/arm-dmc620: Fix dmc620_pmu_irqs_lock/cpu_hotplug_lock circular lock dependency Content-Language: en-GB To: Waiman Long , Will Deacon , Mark Rutland Cc: linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org References: <20230810154114.405742-1-longman@redhat.com> From: Robin Murphy In-Reply-To: <20230810154114.405742-1-longman@redhat.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10/08/2023 4:41 pm, Waiman Long wrote: > The following circular locking dependency was reported when running > cpus online/offline test on an arm64 system. > > [ 84.195923] Chain exists of: > dmc620_pmu_irqs_lock --> cpu_hotplug_lock --> cpuhp_state-down > > [ 84.207305] Possible unsafe locking scenario: > > [ 84.213212] CPU0 CPU1 > [ 84.217729] ---- ---- > [ 84.222247] lock(cpuhp_state-down); > [ 84.225899] lock(cpu_hotplug_lock); > [ 84.232068] lock(cpuhp_state-down); > [ 84.238237] lock(dmc620_pmu_irqs_lock); > [ 84.242236] > *** DEADLOCK *** > > The problematic locking order seems to be > > lock(dmc620_pmu_irqs_lock) --> lock(cpu_hotplug_lock) > > This locking order happens when dmc620_pmu_get_irq() calls > cpuhp_state_add_instance_nocalls(). Since dmc620_pmu_irqs_lock is used > for protecting the dmc620_pmu_irqs structure, we don't actually need > to hold the lock when adding a new instance to the CPU hotplug subsystem. > > Fix this possible deadlock scenario by adding a new dmc620_pmu_get_lock > for protecting the call to __dmc620_pmu_get_irq(). While at it, rename > dmc620_pmu_irqs_lock to dmc620_pmu_list_lock as it is now just protecting > the iteration and modification of pmus_node and irqs_node lists. > > As a result, cpuhp_state_add_instance_nocalls() won't be called with > reanemd dmc620_pmu_list_lock held and cpu_hotplug_lock won't be acquired > after dmc620_pmu_list_lock. > > Suggested-by: Robin Murphy > Signed-off-by: Waiman Long > --- > drivers/perf/arm_dmc620_pmu.c | 34 ++++++++++++++++++++++------------ > 1 file changed, 22 insertions(+), 12 deletions(-) > > diff --git a/drivers/perf/arm_dmc620_pmu.c b/drivers/perf/arm_dmc620_pmu.c > index 9d0f01c4455a..a5bfc8f2e6ab 100644 > --- a/drivers/perf/arm_dmc620_pmu.c > +++ b/drivers/perf/arm_dmc620_pmu.c > @@ -66,8 +66,14 @@ > #define DMC620_PMU_COUNTERn_OFFSET(n) \ > (DMC620_PMU_COUNTERS_BASE + 0x28 * (n)) > > +/* > + * The allowable lock ordering is: > + * - dmc620_pmu_get_lock (protects call to __dmc620_pmu_get_irq()) > + * - dmc620_pmu_list_lock (protects pmus_node & irqs_node lists) Sorry, this isn't right: touching the irqs_node list *is* the aspect of __dmc620_pmu_get_irq() which warrants globally locking. It's then the pmus_node lists which want locking separately from that - those could strictly be locked per dmc620_pmu_irq instance, but that would be a big waste of space, so we can still combine them under a single global lock. I just went too far in thinking I could get away with (ab)using the same lock for both purposes since they didn't overlap :) > + */ > +static DEFINE_MUTEX(dmc620_pmu_get_lock); > +static DEFINE_MUTEX(dmc620_pmu_list_lock); > static LIST_HEAD(dmc620_pmu_irqs); > -static DEFINE_MUTEX(dmc620_pmu_irqs_lock); > > struct dmc620_pmu_irq { > struct hlist_node node; > @@ -423,9 +429,11 @@ static struct dmc620_pmu_irq *__dmc620_pmu_get_irq(int irq_num) > struct dmc620_pmu_irq *irq; > int ret; > > + mutex_lock(&dmc620_pmu_list_lock); > list_for_each_entry(irq, &dmc620_pmu_irqs, irqs_node) > if (irq->irq_num == irq_num && refcount_inc_not_zero(&irq->refcount)) > - return irq; > + goto unlock_out; > + mutex_unlock(&dmc620_pmu_list_lock); > > irq = kzalloc(sizeof(*irq), GFP_KERNEL); > if (!irq) > @@ -452,8 +460,10 @@ static struct dmc620_pmu_irq *__dmc620_pmu_get_irq(int irq_num) > goto out_free_irq; > > irq->irq_num = irq_num; > + mutex_lock(&dmc620_pmu_list_lock); > list_add(&irq->irqs_node, &dmc620_pmu_irqs); > - > +unlock_out: > + mutex_unlock(&dmc620_pmu_list_lock); > return irq; > > out_free_irq: > @@ -467,17 +477,17 @@ static int dmc620_pmu_get_irq(struct dmc620_pmu *dmc620_pmu, int irq_num) > { > struct dmc620_pmu_irq *irq; > > - mutex_lock(&dmc620_pmu_irqs_lock); > + mutex_lock(&dmc620_pmu_get_lock); > irq = __dmc620_pmu_get_irq(irq_num); > - mutex_unlock(&dmc620_pmu_irqs_lock); > + mutex_unlock(&dmc620_pmu_get_lock); > > if (IS_ERR(irq)) > return PTR_ERR(irq); > > dmc620_pmu->irq = irq; > - mutex_lock(&dmc620_pmu_irqs_lock); > + mutex_lock(&dmc620_pmu_list_lock); > list_add_rcu(&dmc620_pmu->pmus_node, &irq->pmus_node); > - mutex_unlock(&dmc620_pmu_irqs_lock); > + mutex_unlock(&dmc620_pmu_list_lock); > > return 0; > } > @@ -486,16 +496,16 @@ static void dmc620_pmu_put_irq(struct dmc620_pmu *dmc620_pmu) > { > struct dmc620_pmu_irq *irq = dmc620_pmu->irq; > > - mutex_lock(&dmc620_pmu_irqs_lock); > + mutex_lock(&dmc620_pmu_list_lock); > list_del_rcu(&dmc620_pmu->pmus_node); Ah, it might be the laziness in this function that's misled you. Logically it ought to just be a case of dropping pmu_list_lock here after removing from the pmus_node list, then taking pmu_get_lock before the following list_del from the main global list (I think that shouldn't *need* to cover the refcount operation as well, but equally there's probably no harm if it does). Thanks, Robin. > if (!refcount_dec_and_test(&irq->refcount)) { > - mutex_unlock(&dmc620_pmu_irqs_lock); > + mutex_unlock(&dmc620_pmu_list_lock); > return; > } > > list_del(&irq->irqs_node); > - mutex_unlock(&dmc620_pmu_irqs_lock); > + mutex_unlock(&dmc620_pmu_list_lock); > > free_irq(irq->irq_num, irq); > cpuhp_state_remove_instance_nocalls(cpuhp_state_num, &irq->node); > @@ -638,10 +648,10 @@ static int dmc620_pmu_cpu_teardown(unsigned int cpu, > return 0; > > /* We're only reading, but this isn't the place to be involving RCU */ > - mutex_lock(&dmc620_pmu_irqs_lock); > + mutex_lock(&dmc620_pmu_list_lock); > list_for_each_entry(dmc620_pmu, &irq->pmus_node, pmus_node) > perf_pmu_migrate_context(&dmc620_pmu->pmu, irq->cpu, target); > - mutex_unlock(&dmc620_pmu_irqs_lock); > + mutex_unlock(&dmc620_pmu_list_lock); > > WARN_ON(irq_set_affinity(irq->irq_num, cpumask_of(target))); > irq->cpu = target;