From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jason Yan Subject: Re: [PATCH] scsi: fix race condition when removing target Date: Thu, 30 Nov 2017 09:18:47 +0800 Message-ID: <5A1F5C77.5050405@huawei.com> References: <20171129030556.47833-1-yanaijie@huawei.com> <1511972310.2671.7.camel@wdc.com> <20171129162050.GA32071@lst.de> <1511977145.2671.13.camel@wdc.com> Mime-Version: 1.0 Content-Type: text/plain; charset="utf-8"; format=flowed Content-Transfer-Encoding: 7bit Return-path: Received: from szxga05-in.huawei.com ([45.249.212.191]:11481 "EHLO szxga05-in.huawei.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752426AbdK3BY6 (ORCPT ); Wed, 29 Nov 2017 20:24:58 -0500 In-Reply-To: <1511977145.2671.13.camel@wdc.com> Sender: linux-scsi-owner@vger.kernel.org List-Id: linux-scsi@vger.kernel.org To: Bart Van Assche , "hch@lst.de" Cc: "zhaohongjiang@huawei.com" , "jthumshirn@suse.de" , "martin.petersen@oracle.com" , "hare@suse.de" , "linux-scsi@vger.kernel.org" , "gregkh@linuxfoundation.org" , "jejb@linux.vnet.ibm.com" , "miaoxie@huawei.com" On 2017/11/30 1:39, Bart Van Assche wrote: > On Wed, 2017-11-29 at 17:20 +0100, hch@lst.de wrote: >> On Wed, Nov 29, 2017 at 04:18:30PM +0000, Bart Van Assche wrote: >>> As the above patch description shows it can happen that the SCSI core calls >>> get_device() after the device reference count has reached zero and before >>> the memory for struct device is freed. Although the above patch looks fine >>> to me, would you consider it acceptable to modify get_device() such that it >>> uses kobject_get_unless_zero() instead of kobject_get()? I'm asking this >>> because that change would help to reduce the complexity of the already too >>> complicated SCSI core. >> >> I don't think we can just modify get_device, but we can add a new >> get_device_unless_zero. In fact I have an open coded variant of that >> in nvme, and was planning to submit one for the current merge window.. > > Sorry but I don't see why we can't modify get_device()? Can you explain why > you think that something like the patch below is wrong? > > Thanks, > > Bart. > Hi Bart, I chose the approach in my patch because it has been used in scsi_device_get() for years and been proved safe. I think using kobject_get_unless_zero() is safe here and can fix this issue too. And this approach is beneficial to all users. > > [PATCH] Make it safe to use get_device() if the reference count is zero > > --- > drivers/base/core.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/drivers/base/core.c b/drivers/base/core.c > index 110230d86527..049a5d9dba8a 100644 > --- a/drivers/base/core.c > +++ b/drivers/base/core.c > @@ -1916,7 +1916,7 @@ EXPORT_SYMBOL_GPL(device_register); > */ > struct device *get_device(struct device *dev) > { > - return dev ? kobj_to_dev(kobject_get(&dev->kobj)) : NULL; > + return dev && kobject_get_unless_zero(&dev->kobj) ? dev : NULL; > } > EXPORT_SYMBOL_GPL(get_device); >