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 1E42EC43334 for ; Fri, 1 Jul 2022 10:22:20 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235700AbiGAKWS (ORCPT ); Fri, 1 Jul 2022 06:22:18 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:43516 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235453AbiGAKWO (ORCPT ); Fri, 1 Jul 2022 06:22:14 -0400 Received: from mailgw01.mediatek.com (unknown [60.244.123.138]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 47FB81AD95; Fri, 1 Jul 2022 03:22:09 -0700 (PDT) X-UUID: a5522599da6d4c31bdfc88bb9124a05f-20220701 X-CID-P-RULE: Release_Ham X-CID-O-INFO: VERSION:1.1.7,REQID:ea4016f1-05ae-49cc-a921-9ce5a8d58047,OB:0,LO B:0,IP:0,URL:5,TC:0,Content:0,EDM:0,RT:0,SF:0,FILE:0,RULE:Release_Ham,ACTI ON:release,TS:5 X-CID-META: VersionHash:87442a2,CLOUDID:833a5cd6-5d6d-4eaf-a635-828a3ee48b7c,C OID:IGNORED,Recheck:0,SF:nil,TC:nil,Content:0,EDM:-3,IP:nil,URL:1,File:nil ,QS:nil,BEC:nil,COL:0 X-UUID: a5522599da6d4c31bdfc88bb9124a05f-20220701 Received: from mtkmbs11n2.mediatek.inc [(172.21.101.187)] by mailgw01.mediatek.com (envelope-from ) (Generic MTA with TLSv1.2 ECDHE-RSA-AES256-GCM-SHA384 256/256) with ESMTP id 1922674619; Fri, 01 Jul 2022 18:21:57 +0800 Received: from mtkcas11.mediatek.inc (172.21.101.40) by mtkmbs11n2.mediatek.inc (172.21.101.187) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384) id 15.2.792.3; Fri, 1 Jul 2022 18:21:55 +0800 Received: from [172.21.84.99] (172.21.84.99) by mtkcas11.mediatek.inc (172.21.101.73) with Microsoft SMTP Server id 15.0.1497.2 via Frontend Transport; Fri, 1 Jul 2022 18:21:55 +0800 Subject: Re: [PATCH v1] PM-runtime: Check supplier_preactivated before release supplier To: "Rafael J. Wysocki" CC: "Rafael J. Wysocki" , Greg Kroah-Hartman , Linux PM , LKML References: <20220613120755.14306-1-peter.wang@mediatek.com> <12028598.O9o76ZdvQC@kreacher> <90b5f619-2dd6-817b-fe2d-f895be0b5b98@mediatek.com> From: Peter Wang Message-ID: <7c3ec17e-296f-ab15-6055-fd59a7d8f036@mediatek.com> Date: Fri, 1 Jul 2022 18:21:55 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset="utf-8"; format=flowed Content-Transfer-Encoding: 7bit Content-Language: en-US Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 7/1/22 12:28 AM, Rafael J. Wysocki wrote: > On Thu, Jun 30, 2022 at 5:19 PM Peter Wang wrote: >> >> On 6/30/22 10:47 PM, Rafael J. Wysocki wrote: >>> On Thu, Jun 30, 2022 at 4:26 PM Peter Wang wrote: >>>> On 6/30/22 12:01 AM, Rafael J. Wysocki wrote: >>>>> [Add CCs to linix-pm, LKML and Greg] >>>>> >>>>> On Wednesday, June 29, 2022 5:32:00 PM CEST Rafael J. Wysocki wrote: >>>>>> On Wed, Jun 29, 2022 at 4:47 PM Peter Wang wrote: >>>>>>> On 6/29/22 9:22 PM, Rafael J. Wysocki wrote: >>>>>>>> On Wed, Jun 29, 2022 at 5:02 AM Peter Wang wrote: >>>>>>>>> On 6/28/22 11:54 PM, Rafael J. Wysocki wrote: >>>>>>>>>> On Tue, Jun 28, 2022 at 3:53 AM Peter Wang wrote: >>>>>>>>>>> On 6/28/22 3:00 AM, Rafael J. Wysocki wrote: >>>>>>>>>>>> On Mon, Jun 13, 2022 at 2:08 PM wrote: >>>>>>>>>>>>> From: Peter Wang >>>>>>>>>>>>> >>>>>>>>>>>>> With divice link of DL_FLAG_PM_RUNTIME, if consumer call pm_runtime_get_suppliers >>>>>>>>>>>>> to prevent supplier enter suspend, pm_runtime_release_supplier should >>>>>>>>>>>>> check supplier_preactivated before let supplier enter suspend. >>>>>>>>>>>> Why? >>>>>>>>>>> because supplier_preactivated is true means supplier cannot enter >>>>>>>>>>> suspend, right? >>>>>>>>>> No, it doesn't mean that. >>>>>>>>> Hi Rafael, >>>>>>>>> >>>>>>>>> if supplier_preactivated is true, means someone call >>>>>>>>> pm_runtime_get_suppliers and >>>>>>>>> before pm_runtime_put_suppliers right? This section suppliers should not >>>>>>>>> enter suspend. >>>>>>>> No, this is not how this is expected to work. >>>>>>>> >>>>>>>> First off, the only caller of pm_runtime_get_suppliers() and >>>>>>>> pm_runtime_put_suppliers() is __driver_probe_device(). Really nobody >>>>>>>> else has any business that would require calling them. >>>>>>> Hi Rafael, >>>>>>> >>>>>>> Yes, you are right! >>>>>>> __driver_probe_device the only one use and just because >>>>>>> __driver_probe_device use >>>>>>> pm_runtime_get_suppliers cause problem. >>>>>>> >>>>>>> >>>>>>>> Second, the role of pm_runtime_get_suppliers() is to "preactivate" the >>>>>>>> suppliers before running probe for a consumer device and the role of >>>>>>> the role of pm_runtime_get_suppliers() is to "preactivate" the suppliers, >>>>>>> but suppliers may suspend immediately after preactivate right? >>>>>>> Here is just this case. this is first racing point. >>>>>>> Thread A: pm_runtime_get_suppliers -> __driver_probe_device >>>>>>> Thread B: pm_runtime_release_supplier >>>>>>> Thread A: Run with supplier not preactivate -> __driver_probe_device >>>>>>> >>>>>>>> pm_runtime_put_suppliers() is to do the cleanup in case the device is >>>>>>>> left in suspend after probing. >>>>>>>> >>>>>>>> IOW, pm_runtime_get_suppliers() is to ensure that the suppliers will >>>>>>>> be active until the probe callback takes over and the rest depends on >>>>>>>> that callback. >>>>>>> The problem of this racing will finally let consumer is active but >>>>>>> supplier is suspended. >>>>>> So it would be better to send a bug report regarding this. >>>>>> >>>>>>> The link relation is broken. >>>>>>> I know you may curious how it happened? right? >>>>>>> Honestly, I am not sure, but I think the second racing point >>>>>>> is rpm_get_suppliers and pm_runtime_put_suppliers(release rpm_active). >>>>>> I'm not sure what you mean by "the racing point". >>>>>> >>>>>> Yes, these functions can run concurrently. >>>>>> >>>>>>> So, I try to fix the first racing point and the problem is gone. >>>>>>> It is full meet expect, and the pm runtime will work smoothly after >>>>>>> __driver_probe_device done. >>>>>> I'm almost sure that there is at least one scenario that would be >>>>>> broken by this change. >>>>> That said, the code in there may be a bit overdesigned. >>>>> >>>>> Does the patch below help? >>>>> >>>>> --- >>>>> drivers/base/power/runtime.c | 14 +------------- >>>>> 1 file changed, 1 insertion(+), 13 deletions(-) >>>>> >>>>> Index: linux-pm/drivers/base/power/runtime.c >>>>> =================================================================== >>>>> --- linux-pm.orig/drivers/base/power/runtime.c >>>>> +++ linux-pm/drivers/base/power/runtime.c >>>>> @@ -1768,7 +1768,6 @@ void pm_runtime_get_suppliers(struct dev >>>>> if (link->flags & DL_FLAG_PM_RUNTIME) { >>>>> link->supplier_preactivated = true; >>>>> pm_runtime_get_sync(link->supplier); >>>>> - refcount_inc(&link->rpm_active); >>>>> } >>>>> >>>>> device_links_read_unlock(idx); >>>>> @@ -1788,19 +1787,8 @@ void pm_runtime_put_suppliers(struct dev >>>>> list_for_each_entry_rcu(link, &dev->links.suppliers, c_node, >>>>> device_links_read_lock_held()) >>>>> if (link->supplier_preactivated) { >>>>> - bool put; >>>>> - >>>>> link->supplier_preactivated = false; >>>>> - >>>>> - spin_lock_irq(&dev->power.lock); >>>>> - >>>>> - put = pm_runtime_status_suspended(dev) && >>>>> - refcount_dec_not_one(&link->rpm_active); >>>>> - >>>>> - spin_unlock_irq(&dev->power.lock); >>>>> - >>>>> - if (put) >>>>> - pm_runtime_put(link->supplier); >>>>> + pm_runtime_put(link->supplier); >>>>> } >>>>> >>>>> device_links_read_unlock(idx); >>>> Hi Rafael, >>>> >>>> I think this patch solve the rpm_active racing problem. >>>> But it still have problem that >>>> pm_runtime_get_suppliers call pm_runtime_get_sync(link->supplier) >>>> and supplier could suspend immediately by other thread who call >>>> pm_runtime_release_supplier. >>> No, it won't, because pm_runtime_release_supplier() won't drop the >>> reference on the supplier taken by pm_runtime_get_suppliers(0 after >>> the patch. >> Hi Rafael, >> >> I think pm_runtime_release_supplier will always decrese the reference >> rpm_active count to 1 and check idle will let supplier enter suspend. Am >> I wrong? >> >> Could you explain why this patch won't drop the reference? > What matters is the supplier's PM-runtime usage counter and (with the > patch above applied) pm_runtime_get_suppliers() bumps it up via > pm_runtime_get_sync() and it doesn't bump up the device link's > rpm_active count at the same time. > > This is important, because the number of times > pm_runtime_release_supplier() decrements the supplier's usage counter > is the same as the rpm_active count value at the beginning of that > function minus 1. Now, rpm_active is 1 initially and every time it > gets incremented, the supplier's usage counter is also incremented. > Combined with the observation in the previous paragraph, this means > that after pm_runtime_get_suppliers() the value of the supplier's > PM-runtime usage counter will always be greater than the rpm_active > value minus 1, so pm_runtime_release_supplier() cannot decrement it > down to zero until pm_runtime_put_suppliers() runs. Hi Rafael, Yes, it is very clear! I miss this important key point that usage_count is always > rpm_active 1. I think this patch could work. Thanks. Peter