From mboxrd@z Thu Jan 1 00:00:00 1970 Return-path: Received: from moutng.kundenserver.de ([212.227.17.8]:57033 "EHLO moutng.kundenserver.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751246AbaAQO2M (ORCPT ); Fri, 17 Jan 2014 09:28:12 -0500 From: Arnd Bergmann To: Hans Verkuil Subject: Re: [PATCH, RFC 07/30] [media] radio-cadet: avoid interruptible_sleep_on race Date: Fri, 17 Jan 2014 15:28:01 +0100 Cc: linux-kernel@vger.kernel.org, Mauro Carvalho Chehab , linux-media@vger.kernel.org References: <1388664474-1710039-1-git-send-email-arnd@arndb.de> <1388664474-1710039-8-git-send-email-arnd@arndb.de> <52D90A2F.2030903@xs4all.nl> In-Reply-To: <52D90A2F.2030903@xs4all.nl> MIME-Version: 1.0 Content-Type: Text/Plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Message-Id: <201401171528.02016.arnd@arndb.de> Sender: linux-media-owner@vger.kernel.org List-ID: On Friday 17 January 2014, Hans Verkuil wrote: > > @@ -323,25 +324,32 @@ static ssize_t cadet_read(struct file *file, char __user *data, size_t count, lo > > struct cadet *dev = video_drvdata(file); > > unsigned char readbuf[RDS_BUFFER]; > > int i = 0; > > + DEFINE_WAIT(wait); > > > > mutex_lock(&dev->lock); > > if (dev->rdsstat == 0) > > cadet_start_rds(dev); > > - if (dev->rdsin == dev->rdsout) { > > + while (1) { > > + prepare_to_wait(&dev->read_queue, &wait, TASK_INTERRUPTIBLE); > > + if (dev->rdsin != dev->rdsout) > > + break; > > + > > if (file->f_flags & O_NONBLOCK) { > > i = -EWOULDBLOCK; > > goto unlock; > > } > > mutex_unlock(&dev->lock); > > - interruptible_sleep_on(&dev->read_queue); > > + schedule(); > > mutex_lock(&dev->lock); > > } > > + > > This seems overly complicated. Isn't it enough to replace interruptible_sleep_on > by 'wait_event_interruptible(&dev->read_queue, dev->rdsin != dev->rdsout);'? > > Or am I missing something subtle? The existing code sleeps with &dev->lock released because the cadet_handler() function needs to grab (and release) the same lock before it can wake up the reader thread. Doing the simple wait_event_interruptible() would result in a deadlock here. Arnd