Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751562AbdFIKgp (ORCPT ); Fri, 9 Jun 2017 06:36:45 -0400 Received: from mail-oi0-f66.google.com ([209.85.218.66]:36680 "EHLO mail-oi0-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751519AbdFIKgn (ORCPT ); Fri, 9 Jun 2017 06:36:43 -0400 MIME-Version: 1.0 In-Reply-To: References: <1496916298-5909-1-git-send-email-binoy.jayan@linaro.org> <1496916298-5909-2-git-send-email-binoy.jayan@linaro.org> From: Arnd Bergmann Date: Fri, 9 Jun 2017 12:36:41 +0200 X-Google-Sender-Auth: 40_YuGWRAz-s2b6BYj5xBiulrIY Message-ID: Subject: Re: [PATCH 1/3] media: ngene: Replace semaphore cmd_mutex with mutex To: Binoy Jayan Cc: Linux Kernel Mailing List , Rajendra , Mark Brown , Mauro Carvalho Chehab , Sakari Ailus , Julia Lawall , "Michael S. Tsirkin" , Cao jin , Linux Media Mailing List Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2459 Lines: 60 On Fri, Jun 9, 2017 at 6:37 AM, Binoy Jayan wrote: > On 8 June 2017 at 20:40, Arnd Bergmann wrote: >> On Thu, Jun 8, 2017 at 12:04 PM, Binoy Jayan wrote: >>> The semaphore 'cmd_mutex' is used as a simple mutex, so >>> it should be written as one. Semaphores are going away in the future. >>> >>> Signed-off-by: Binoy Jayan >>> --- >> >>> @@ -1283,7 +1283,7 @@ static int ngene_load_firm(struct ngene *dev) >>> >>> static void ngene_stop(struct ngene *dev) >>> { >>> - down(&dev->cmd_mutex); >>> + mutex_lock(&dev->cmd_mutex); >>> i2c_del_adapter(&(dev->channel[0].i2c_adapter)); >>> i2c_del_adapter(&(dev->channel[1].i2c_adapter)); >>> ngwritel(0, NGENE_INT_ENABLE); >> >> Are you sure about this one? There is only one mutex_lock() and >> then the structure gets freed without a corresponding mutex_unlock(). >> >> I suspect this violates some rules of mutexes, either when compile >> testing with "make C=1", or when running with lockdep enabled. >> >> Can we actually have a concurrently held mutex at the time we >> get here? If not, using mutex_destroy() in place of the down() >> may be the right answer. > > I noticed the missing 'up' here, but may be semaphores do not have > to adhere to that rule? The rules for semaphores are very lax, the up() and down() may be in completely separate contexts, the up() can even happen from an interrupt handler IIRC. I read up on the sparse annotations now and found that it only tracks spinlocks and rwlocks using the __acquires() annotation, but not semaphores or mutexes. I'm still not sure whether lockdep requires the mutex to be released before it gets freed, the code may actually be fine, but it does seem odd. > Thank you for pointing out that. I'll check the > concurrency part. By the way why do we need mutex_destoy? > To debug an aberrate condition? At first I suspected the down() here was added for the same purpose as a mutex_destroy: to ensure that we are in a sane state before we free the device structure, but the way they achieve that is completely different. However, if there is any way that a command may still be in progress by the time we get to ngene_stop(), we may also be lacking reference counting on the ngene structure here. So far I haven't found any of those, and think the mutex_destroy() is sufficient here as a debugging help. Arnd