From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756833Ab0KONlG (ORCPT ); Mon, 15 Nov 2010 08:41:06 -0500 Received: from tango.tkos.co.il ([62.219.50.35]:34892 "EHLO tango.tkos.co.il" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756772Ab0KONlD (ORCPT ); Mon, 15 Nov 2010 08:41:03 -0500 Date: Mon, 15 Nov 2010 15:40:26 +0200 From: Baruch Siach To: Jiri Slaby Cc: linux-kernel@vger.kernel.org, Andrew Morton , Indan Zupancic , Greg KH , "H. Peter Anvin" , Alex Gershgorin Subject: Re: [PATCHv3] drivers/misc: Altera active serial implementation Message-ID: <20101115134025.GB3649@jasper.tkos.co.il> References: <4CE0FE71.6000303@suse.cz> <20101115100838.GA3649@jasper.tkos.co.il> <4CE10B32.8090705@suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4CE10B32.8090705@suse.cz> User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Jiri, On Mon, Nov 15, 2010 at 11:28:02AM +0100, Jiri Slaby wrote: > On 11/15/2010 11:08 AM, Baruch Siach wrote: > > Thanks for reviewing. See my response to your comments below. > > > > On Mon, Nov 15, 2010 at 10:33:37AM +0100, Jiri Slaby wrote: > >> On 11/15/2010 09:23 AM, Baruch Siach wrote: > >>> --- /dev/null > >>> +++ b/drivers/misc/altera_as.c > >>> @@ -0,0 +1,349 @@ > >> ... > >>> +struct altera_as_device { > >>> + unsigned id; > >>> + struct device *dev; > >>> + struct miscdevice miscdev; > >>> + struct mutex open_lock; > >>> + struct gpio gpios[AS_GPIO_NUM]; > >>> +}; > >>> + > >>> +/* > >>> + * The only functions updating this array are .probe/.remove, which are > >>> + * serialized. Hence, no locking is needed here. > >>> + */ > >>> +static struct { > >>> + int minor; > >> > >> Why you need minor here? It's in drvdata->miscdev.minor. > > > > I use the minor field to lookup the drvdata pointer in get_as_dev(), which I > > use is altera_as_open(). I do this because I have no access to the 'struct > > device' pointer from the .open method. Do you know a better way? > > > >>> + struct altera_as_device *drvdata; > >>> +} altera_as_devs[AS_MAX_DEVS]; > >> > >> You don't need the struct at all. > >> static struct altera_as_device *drvdata[AS_MAX_DEVS]; > >> should be enough. > > > > See above. > > The answer to you previous question is here. You can just have a global > array of struct altera_as_device. This static array would occupy sizeof(struct altera_as_device) * AS_MAX_DEVS == 128 (for 32bit) * 16 == 2KB unconditionally. This seems unreasonable to me. IMO struct altera_as_device should be allocated per device at run time. > >>> +static int altera_as_open(struct inode *inode, struct file *file) > >>> +{ > >>> + int ret; > >>> + struct altera_as_device *drvdata = get_as_dev(iminor(inode)); > >>> + > >>> + if (drvdata == NULL) > >>> + return -ENODEV; > >>> + file->private_data = drvdata; > >>> + > >>> + ret = mutex_trylock(&drvdata->open_lock); > >>> + if (ret == 0) > >>> + return -EBUSY; > >>> + > >>> + ret = gpio_request_array(drvdata->gpios, ARRAY_SIZE(drvdata->gpios)); > >>> + if (ret < 0) { > >>> + mutex_unlock(&drvdata->open_lock); > >>> + return ret; > >>> + } > >> > >> So leaving to userspace with mutex held. This is *wrong*. use > >> test_and_set_bit or alike instead. > > > > Can you explain why this is wrong? I don't mind doing test_and_set_bit > > instead, I'm just curious. > > The mutex has an owner which it expects to unlock it. For example if you > fork a process which already opened the device, you have a problem. This > is a technical point. Another point is that it's ugly to leave to > userspace with any lock. > > >>> +static int __init altera_as_probe(struct platform_device *pdev) > >>> +{ > >> ... > >>> + drvdata->miscdev.minor = MISC_DYNAMIC_MINOR; > >>> + drvdata->miscdev.name = kasprintf(GFP_KERNEL, "altera_as%d", > >>> + pdata->id); > >>> + if (drvdata->miscdev.name == NULL) > >>> + return -ENOMEM; > >>> + drvdata->miscdev.fops = &altera_as_fops; > >>> + if (misc_register(&drvdata->miscdev) < 0) { > >> > >> Hmm, how many devices can be in the system? > > > > Up to AS_MAX_DEVS, currently 16. > > > >> This doesn't look like the right way. > > > > What is the right way then? > > Ok, so for that count it definitely deserves its own major to not eat up > misc device space. A previous version of this driver[1] did just that. It was Greg KH who suggested[2] to use the misc class. [1] http://article.gmane.org/gmane.linux.kernel/1057434 [2] http://article.gmane.org/gmane.linux.kernel/1058627 > >>> + kfree(drvdata->miscdev.name); > >>> + return -ENODEV; > >>> + } > >>> + altera_as_devs[pdata->id].minor = drvdata->miscdev.minor; > >>> + altera_as_devs[pdata->id].drvdata = drvdata; > >> > >> So now the device is usable without the mutex and gpios inited? > > > > I was thinking that the device lock which is being held during the .probe run, > > prevents the device from being open. After all I can still return -EGOAWAY at > > this point. Am I wrong? > > > > If so, I'll reorder this code. > > This cannot be done easily. You need to set drvdata prior to minor and > after all the assignments here. The former because in get_as_dev you > test minor and return drvdata. When drvdata is not set it contains NULL. The .open method then just returns -ENODEV. So moving the drvdata assignment to the end of .probe should plug all race scenarios without any additional locking, isn't it? > The latter because you use open_lock & > gpios after playing with minor & drvdata. > > Technically, it can be done with a use of barriers, but I don't > recommend it as drivers should not use barriers at all. You should > introduce some locking here (it introduces barriers on its own). > > So move the assignment to altera_as_devs below the mutex_init & gpios > setup and lock it appropriately. Then add a lock to get_as_dev. > > >>> + mutex_init(&drvdata->open_lock); > >>> + > >>> + drvdata->id = pdata->id; > >>> + > >>> + drvdata->gpios[ASIO_DATA].gpio = pdata->data; > >>> + drvdata->gpios[ASIO_DATA].flags = GPIOF_IN; > >>> + drvdata->gpios[ASIO_DATA].label = "as_data"; > >> ... > >>> + drvdata->gpios[ASIO_NCE].gpio = pdata->nce; > >>> + drvdata->gpios[ASIO_NCE].flags = GPIOF_OUT_INIT_HIGH; > >>> + drvdata->gpios[ASIO_NCE].label = "as_nce"; > >>> + > >>> + dev_info(drvdata->dev, "Altera Cyclone Active Serial driver\n"); > >>> + > >>> + return 0; > >>> +} > > regards, > -- > js > suse labs baruch -- ~. .~ Tk Open Systems =}------------------------------------------------ooO--U--Ooo------------{= - baruch@tkos.co.il - tel: +972.2.679.5364, http://www.tkos.co.il -