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 A523FC001DB for ; Fri, 4 Aug 2023 16:43:37 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230384AbjHDQnI (ORCPT ); Fri, 4 Aug 2023 12:43:08 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:51360 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230216AbjHDQmd (ORCPT ); Fri, 4 Aug 2023 12:42:33 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id A7A1F46B1 for ; Fri, 4 Aug 2023 09:41:48 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1691167307; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=TpRfp9EkcGgDnA00HaJLJh34HEC2xPXrwDbKDYV99NU=; b=L9aRN6obhWfmm99Srf5aT2vtAgMc9NlWlD3ZCJh+eP3kzBqnJ/J0DJVG9lOIaQ7kBuNbNS Q2a1+tRj8HcYUfYaOp+CcBfRAEftYBUVnZB+CEAsqoD5y8swfQwxd1OhTzQ27qx2bP2CIS ZPTsajsdcHCLo+Tl0e7J53GNsyoehno= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-624-ATNWP6w7P_KUXrB3W42QvA-1; Fri, 04 Aug 2023 12:41:43 -0400 X-MC-Unique: ATNWP6w7P_KUXrB3W42QvA-1 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.rdu2.redhat.com [10.11.54.5]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 6E9DC185A792; Fri, 4 Aug 2023 16:41:43 +0000 (UTC) Received: from [10.22.33.115] (unknown [10.22.33.115]) by smtp.corp.redhat.com (Postfix) with ESMTP id 19940F7FB7; Fri, 4 Aug 2023 16:41:43 +0000 (UTC) Message-ID: <8a5987c4-42f8-2f71-8135-eafedf4a3ef2@redhat.com> Date: Fri, 4 Aug 2023 12:41:42 -0400 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.7.1 Subject: Re: [PATCH v3] perf/arm-dmc620: Fix dmc620_pmu_irqs_lock/cpu_hotplug_lock circular lock dependency Content-Language: en-US To: Will Deacon Cc: Mark Rutland , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, Robin Murphy References: <20230722031729.3913953-1-longman@redhat.com> <20230728150614.GF21718@willie-the-truck> <62d4b353-0237-9ec6-a63e-8a7a6764aba5@redhat.com> <3e6a9ca3-3be5-8207-4923-8ecd141c04eb@redhat.com> <20230804162954.GD30679@willie-the-truck> From: Waiman Long In-Reply-To: <20230804162954.GD30679@willie-the-truck> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 3.1 on 10.11.54.5 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 8/4/23 12:29, Will Deacon wrote: > On Wed, Aug 02, 2023 at 09:44:58PM -0400, Waiman Long wrote: >> On 8/2/23 21:37, Waiman Long wrote: >>> On 7/28/23 11:06, Will Deacon wrote: >>>> On Fri, Jul 21, 2023 at 11:17:28PM -0400, 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() is called from >>>>> dmc620_pmu_device_probe(). Since dmc620_pmu_irqs_lock is used for >>>>> protecting the dmc620_pmu_irqs structure only, 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 releasing the lock before >>>>> calling cpuhp_state_add_instance_nocalls() and reacquiring it >>>>> afterward. >>>>> To avoid the possibility of 2 racing dmc620_pmu_get_irq() calls >>>>> inserting >>>>> duplicated dmc620_pmu_irq structures with the same irq number, a dummy >>>>> entry is inserted before releasing the lock which will block a >>>>> competing >>>>> thread from inserting another irq structure of the same irq number. >>>>> >>>>> Suggested-by: Robin Murphy >>>>> Signed-off-by: Waiman Long >>>>> --- >>>>>   drivers/perf/arm_dmc620_pmu.c | 28 ++++++++++++++++++++++------ >>>>>   1 file changed, 22 insertions(+), 6 deletions(-) >>>>> >>>>> diff --git a/drivers/perf/arm_dmc620_pmu.c >>>>> b/drivers/perf/arm_dmc620_pmu.c >>>>> index 9d0f01c4455a..7cafd4dd4522 100644 >>>>> --- a/drivers/perf/arm_dmc620_pmu.c >>>>> +++ b/drivers/perf/arm_dmc620_pmu.c >>>>> @@ -76,6 +76,7 @@ struct dmc620_pmu_irq { >>>>>       refcount_t refcount; >>>>>       unsigned int irq_num; >>>>>       unsigned int cpu; >>>>> +    unsigned int valid; >>>>>   }; >>>>>     struct dmc620_pmu { >>>>> @@ -423,9 +424,14 @@ static struct dmc620_pmu_irq >>>>> *__dmc620_pmu_get_irq(int irq_num) >>>>>       struct dmc620_pmu_irq *irq; >>>>>       int ret; >>>>>   -    list_for_each_entry(irq, &dmc620_pmu_irqs, irqs_node) >>>>> -        if (irq->irq_num == irq_num && >>>>> refcount_inc_not_zero(&irq->refcount)) >>>>> +    list_for_each_entry(irq, &dmc620_pmu_irqs, irqs_node) { >>>>> +        if (irq->irq_num != irq_num) >>>>> +            continue; >>>>> +        if (!irq->valid) >>>>> +            return ERR_PTR(-EAGAIN);    /* Try again later */ >>>> It looks like this can bubble up to the probe() routine. Does the driver >>>> core handle -EAGAIN coming back from a probe routine? >>> Right, I should add code to handle this error condition. I think it can >>> be handled in dmc620_pmu_get_irq(). The important thing is to release >>> the mutex, wait a few ms and try again. What do you think? >>>>> +        if (refcount_inc_not_zero(&irq->refcount)) >>>>>               return irq; >>>>> +    } >>>>>         irq = kzalloc(sizeof(*irq), GFP_KERNEL); >>>>>       if (!irq) >>>>> @@ -447,13 +453,23 @@ static struct dmc620_pmu_irq >>>>> *__dmc620_pmu_get_irq(int irq_num) >>>>>       if (ret) >>>>>           goto out_free_irq; >>>>>   -    ret = cpuhp_state_add_instance_nocalls(cpuhp_state_num, >>>>> &irq->node); >>>>> -    if (ret) >>>>> -        goto out_free_irq; >>>>> - >>>>>       irq->irq_num = irq_num; >>>>>       list_add(&irq->irqs_node, &dmc620_pmu_irqs); >>>>>   +    /* >>>>> +     * Release dmc620_pmu_irqs_lock before calling >>>>> +     * cpuhp_state_add_instance_nocalls() and reacquire it afterward. >>>>> +     */ >>>>> +    mutex_unlock(&dmc620_pmu_irqs_lock); >>>>> +    ret = cpuhp_state_add_instance_nocalls(cpuhp_state_num, >>>>> &irq->node); >>>>> +    mutex_lock(&dmc620_pmu_irqs_lock); >>>>> + >>>>> +    if (ret) { >>>>> +        list_del(&irq->irqs_node); >>>>> +        goto out_free_irq; >>>>> +    } >>>>> + >>>>> +    irq->valid = true; >>>> Do you actually need a new flag here, or could we use a refcount of zero >>>> to indicate that the irq descriptor is still being constructed? >>> A refcount of zero can also mean that an existing irq is about to be >>> removed. Right? So I don't think we can use that for this purpose. >>> Besides, there is a 4-byte hole in the structure anyway for arm64. >> Alternatively, I can use a special reference count value, say -1, to signal >> that the irq is not valid yet. What do you think? > If the device is being removed, we should teardown the irq handler first, > so I don't see why the refcount isn't the right thing. According to the current code, a refcount of 0 will cause the caller to skip the entry and eventually create a new irq itself. That may cause the creation of 2 dmc620_pmu_irq structures with the same irq number. Will that be a problem? The reason why I see a problem with a refcount of 0 because it can now signal both the creation of the new irq or the retirement of an old irq that is to be teared down. Cheers, Longman