From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932821AbaHYOEc (ORCPT ); Mon, 25 Aug 2014 10:04:32 -0400 Received: from a.ns.miles-group.at ([95.130.255.143]:65275 "EHLO radon.swed.at" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932300AbaHYOEb (ORCPT ); Mon, 25 Aug 2014 10:04:31 -0400 Message-ID: <53FB426B.90002@nod.at> Date: Mon, 25 Aug 2014 16:04:27 +0200 From: Richard Weinberger User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.6.0 MIME-Version: 1.0 To: Thorsten Knabe CC: linux-kernel@vger.kernel.org, Thomas Meyer , Valdis.Kletnieks@vt.edu Subject: Re: [PATCH] UML: UBD: Fix for processes stuck in D state forever in UserModeLinux References: <53F89B7A.1040406@thorsten-knabe.de> <53F8B48D.8060209@nod.at> <53F8D2A7.70204@thorsten-knabe.de> <53F9D661.8080100@nod.at> <53FA6EF3.3080902@thorsten-knabe.de> <53FB3941.3060504@nod.at> <53FB4168.9070200@thorsten-knabe.de> In-Reply-To: <53FB4168.9070200@thorsten-knabe.de> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Am 25.08.2014 16:00, schrieb Thorsten Knabe: > On 08/25/2014 03:25 PM, Richard Weinberger wrote: >> Am 25.08.2014 01:02, schrieb Thorsten Knabe: >>> On 08/24/2014 02:11 PM, Richard Weinberger wrote: >>>> Am 23.08.2014 19:43, schrieb Thorsten Knabe: >>>>> Hi Richard. >>>>> >>>>> On 08/23/2014 05:34 PM, Richard Weinberger wrote: >>>>>> Hi! >>>>>> >>>>>> Am 23.08.2014 15:47, schrieb Thorsten Knabe: >>>>>>> From: Thorsten Knabe >>>>>>> >>>>>>> UML: UBD: Fix for processes stuck in D state forever in UserModeLinux. >>>>>>> >>>>>>> Starting with Linux 3.12 processes get stuck in D state forever in >>>>>>> UserModeLinux under sync heavy workloads. This bug was introduced by >>>>>>> commit 805f11a0d5 (um: ubd: Add REQ_FLUSH suppport). >>>>>>> Fix bug by adding a check if FLUSH request was successfully submitted to >>>>>>> the I/O thread and keeping the FLUSH request on the request queue on >>>>>>> submission failures. >>>>>>> >>>>>>> Fixes: 805f11a0d5 (um: ubd: Add REQ_FLUSH suppport) >>>>>>> Signed-off-by: Thorsten Knabe >>>>>> >>>>>> Thanks a lot for hunting this issue down. >>>>>> >>>>>>> --- >>>>>>> Patch applies to 3.16.1. >>>>>>> >>>>>>> diff --git a/arch/um/drivers/ubd_kern.c b/arch/um/drivers/ubd_kern.c >>>>>>> index 3716e69..b7d2840 100644 >>>>>>> --- a/arch/um/drivers/ubd_kern.c >>>>>>> +++ b/arch/um/drivers/ubd_kern.c >>>>>>> @@ -1277,7 +1277,7 @@ static void do_ubd_request(struct request_queue *q) >>>>>>> >>>>>>> while(1){ >>>>>>> struct ubd *dev = q->queuedata; >>>>>>> - if(dev->end_sg == 0){ >>>>>>> + if(dev->request == NULL){ >>>>>> >>>>>> Why do we need this specific change? >>>>> >>>>> This change is required, because for FLUSH requests dev->end_sg is >>>>> initialized to 0 by blk_rq_map_sg() a few lines above, as FLUSH requests >>>>> have no data blocks attached to themselves. >>>> >>>> You meant "below"? Looks like I really miss something here. >>>> At the bottom of the while(1) loop we have >>>> dev->end_sg = 0; >>>> dev->request = NULL; >>> >>> No. The problematic line is: >>> dev->end_sg = blk_rq_map_sg(q, req, dev->sg); >>> and blk_rq_map_sg() returning 0 for REQ_FLUSH requests, because they >>> have no associated data blocks. >>> >>> Hence on the next iteration of the while(1) loop: >>> if(dev->end_sg == 0){ >> >> At the bottom of the while loop dev->end_sg will be set to 0 anyway, this is what puzzles >> me so hard. >> (And dev->request too) > > Yes, but the statements: > dev->end_sg = 0; > dev->request = NULL; > at the end of the while loop will only be reached (for FLUSH requests > only with my patch applied) when submit_request() succeeds. Otherwise > the function do_ubd_request() returns early leaving dev->end_sg and > dev->request untouched. Can you please add this info into the commit message, I'm sure others get also confused by that.^^ The logic in do_ubd_request() royal mess which needs fixing too. :-\ Thanks, //richard