Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753247AbaAGF0B (ORCPT ); Tue, 7 Jan 2014 00:26:01 -0500 Received: from mail-ee0-f44.google.com ([74.125.83.44]:65041 "EHLO mail-ee0-f44.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750963AbaAGF0A (ORCPT ); Tue, 7 Jan 2014 00:26:00 -0500 MIME-Version: 1.0 In-Reply-To: References: <52C98BCC.9040900@acm.org> <20140105182640.GA2522@kroah.com> <20140106213111.GA2536@redhat.com> Date: Tue, 7 Jan 2014 13:25:58 +0800 X-Google-Sender-Auth: Sp6KcAcHgxULyy6CnQ8N4PDQt-A Message-ID: Subject: Re: kobject: provide kobject_put_wait to fix module unload race From: Linus Torvalds To: Mikulas Patocka Cc: Mike Snitzer , Greg Kroah-Hartman , Bart Van Assche , Jeff Mahoney , Linux Kernel Mailing List , device-mapper development , Thomas Gleixner , Paul McKenney , Ingo Molnar Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org This looks completely broken to me. You do a "kobject_put()" and then after you've dropped that last use, you wait for the completion of something that may already have been free'd. Wtf? Am I missing something? Linus On Tue, Jan 7, 2014 at 12:01 PM, Mikulas Patocka wrote: > > > On Mon, 6 Jan 2014, Mike Snitzer wrote: > >> On Mon, Jan 06 2014 at 1:55pm -0500, >> Mikulas Patocka wrote: >> >> > >> > >> > On Sun, 5 Jan 2014, Greg Kroah-Hartman wrote: >> > >> > > On Sun, Jan 05, 2014 at 05:43:56PM +0100, Bart Van Assche wrote: >> > > > On 01/04/14 19:06, Mikulas Patocka wrote: >> > > > > - if (t && !t->release) >> > > > > - pr_debug("kobject: '%s' (%p): does not have a release() " >> > > > > - "function, it is broken and must be fixed.\n", >> > > > > - kobject_name(kobj), kobj); >> > > > > - >> > > > >> > > > Has it been considered to issue a warning if no release function has >> > > > been defined and free_completion == NULL instead of removing the above >> > > > debug message entirely ? I think even with this patch applied it is >> > > > still wrong to invoke kobject_put() on an object without defining a >> > > > release function. >> > > >> > > This patch isn't going to be applied, and I've reverted the original >> > > commit, so there shouldn't be any issues anymore with this code. >> > >> > Why? This patch does the same thing as >> > eee031649707db3c9920d9498f8d03819b74fc23, but it's smaller. So why did you >> > accept eee031649707db3c9920d9498f8d03819b74fc23 and not this? >> > >> > The code to wait for kobject destruction using completion already exists >> > in cpufreq_sysfs_release, cpuidle_sysfs_release, >> > cpuidle_state_sysfs_release, cpuidle_driver_sysfs_release, >> > ext4_sb_release, ext4_feat_release, f2fs_sb_release (these are the only >> > kobject users that are correct w.r.t. module unloading), so if you accept >> > this patch, you can simplify them to use kobject_put_wait. >> >> Hi Mikulas, >> >> Please just submit a DM-only patch that follows the same racey pattern >> of firing a completion from the kobj_type .release method in dm_mod. >> I'll get it queued up for 3.14. >> >> If/when we gets reports of a crash due to dm_mod unload racing with >> kobject_put we can revisit this. >> >> Thanks, >> Mike > > Here I'm sending dm-only patch. > > > > dm: wait until kobject is destroyed > > There may be other parts of the kernel taking reference to the dm kobject. > We must wait until they drop the references before deallocating the md > structure. > > Signed-off-by: Mikulas Patocka > Cc: stable@vger.kernel.org > > --- > drivers/md/dm-sysfs.c | 10 +++++++++- > drivers/md/dm.c | 11 +++++++++++ > drivers/md/dm.h | 2 ++ > 3 files changed, 22 insertions(+), 1 deletion(-) > > Index: linux-3.13-rc7/drivers/md/dm-sysfs.c > =================================================================== > --- linux-3.13-rc7.orig/drivers/md/dm-sysfs.c 2014-01-07 02:06:08.000000000 +0100 > +++ linux-3.13-rc7/drivers/md/dm-sysfs.c 2014-01-07 02:07:09.000000000 +0100 > @@ -79,6 +79,11 @@ static const struct sysfs_ops dm_sysfs_o > .show = dm_attr_show, > }; > > +static void dm_kobject_release(struct kobject *kobj) > +{ > + complete(dm_get_completion_from_kobject(kobj)); > +} > + > /* > * dm kobject is embedded in mapped_device structure > * no need to define release function here > @@ -86,6 +91,7 @@ static const struct sysfs_ops dm_sysfs_o > static struct kobj_type dm_ktype = { > .sysfs_ops = &dm_sysfs_ops, > .default_attrs = dm_attrs, > + .release = dm_kobject_release, > }; > > /* > @@ -104,5 +110,7 @@ int dm_sysfs_init(struct mapped_device * > */ > void dm_sysfs_exit(struct mapped_device *md) > { > - kobject_put(dm_kobject(md)); > + struct kobject *kobj = dm_kobject(md); > + kobject_put(kobj); > + wait_for_completion(dm_get_completion_from_kobject(kobj)); > } > Index: linux-3.13-rc7/drivers/md/dm.c > =================================================================== > --- linux-3.13-rc7.orig/drivers/md/dm.c 2014-01-07 02:07:09.000000000 +0100 > +++ linux-3.13-rc7/drivers/md/dm.c 2014-01-07 04:58:37.000000000 +0100 > @@ -203,6 +203,9 @@ struct mapped_device { > /* sysfs handle */ > struct kobject kobj; > > + /* wait until the kobject is released */ > + struct completion kobj_completion; > + > /* zero-length flush that will be cloned and submitted to targets */ > struct bio flush_bio; > > @@ -2049,6 +2052,7 @@ static struct mapped_device *alloc_dev(i > init_waitqueue_head(&md->wait); > INIT_WORK(&md->work, dm_wq_work); > init_waitqueue_head(&md->eventq); > + init_completion(&md->kobj_completion); > > md->disk->major = _major; > md->disk->first_minor = minor; > @@ -2931,6 +2935,13 @@ struct mapped_device *dm_get_from_kobjec > return md; > } > > +struct completion *dm_get_completion_from_kobject(struct kobject *kobj) > +{ > + struct mapped_device *md = container_of(kobj, struct mapped_device, kobj); > + > + return &md->kobj_completion; > +} > + > int dm_suspended_md(struct mapped_device *md) > { > return test_bit(DMF_SUSPENDED, &md->flags); > Index: linux-3.13-rc7/drivers/md/dm.h > =================================================================== > --- linux-3.13-rc7.orig/drivers/md/dm.h 2014-01-07 02:06:08.000000000 +0100 > +++ linux-3.13-rc7/drivers/md/dm.h 2014-01-07 02:07:09.000000000 +0100 > @@ -15,6 +15,7 @@ > #include > #include > #include > +#include > > #include "dm-stats.h" > > @@ -152,6 +153,7 @@ int dm_sysfs_init(struct mapped_device * > void dm_sysfs_exit(struct mapped_device *md); > struct kobject *dm_kobject(struct mapped_device *md); > struct mapped_device *dm_get_from_kobject(struct kobject *kobj); > +struct completion *dm_get_completion_from_kobject(struct kobject *kobj); > > /* > * Targets for linear and striped mappings -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/