From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: Ming Lei <ming.lei@redhat.com>
Cc: Petr Mladek <pmladek@suse.com>,
linux-kernel@vger.kernel.org,
Luis Chamberlain <mcgrof@kernel.org>,
Joe Lawrence <joe.lawrence@redhat.com>
Subject: Re: [PATCH 2/2] kobject: wait until kobject is cleaned up before freeing module
Date: Wed, 10 Nov 2021 08:03:04 +0100 [thread overview]
Message-ID: <YYtuqF2Pj/D9iEBa@kroah.com> (raw)
In-Reply-To: <YYseW96UYRJ/eE5p@T590>
On Wed, Nov 10, 2021 at 09:20:27AM +0800, Ming Lei wrote:
> On Tue, Nov 09, 2021 at 02:14:09PM +0100, Petr Mladek wrote:
> > On Tue 2021-11-09 10:00:27, Ming Lei wrote:
> > > On Mon, Nov 08, 2021 at 06:26:25PM +0100, Petr Mladek wrote:
> > > > On Fri 2021-11-05 14:37:10, Ming Lei wrote:
> > > > > kobject_put() may become asynchronously because of
> > > > > CONFIG_DEBUG_KOBJECT_RELEASE, so once kobject_put() returns, the caller may
> > > > > expect the kobject is released after the last refcnt is dropped, however
> > > > > CONFIG_DEBUG_KOBJECT_RELEASE just schedules one delayed work function
> > > > > for cleaning up the kobject. Inside the cleanup handler, kobj->ktype and
> > > > > kobj->ktype->release are required.
> > > > >
> > > > > It is supposed that no activity is on kobject itself any more since
> > > > > module_exit() is started, so it is reasonable for the kobject user or
> > > > > driver to expect that kobject can be really released in the last run of
> > > > > kobject_put() in module_exit() code path. Otherwise, it can be thought as
> > > > > one driver's bug since the module is going away.
> > > >
> > > > Honestly, this looks a bit fragile. What if there is still another
> > > > reference from some reason. IMHO, it is easy to do it wrong.
> > > > The kobject stuff is super-tricky.
> > > >
> > > > Yes, there is the argument that it is a drivers bug when it does not
> > > > work.
> > >
> > > That is another 'issue'(even not sure if there is really), and it isn't covered
> > > in this patchset, which focuses on fixing CONFIG_DEBUG_KOBJECT_RELEASE, so
> > > please do not mix the two here.
> >
> > Yes, it is another issue but the relation is very important.
> >
> > My understanding is that this patch prevents problems caused by
> > the delayed work. The kobject is added into kobj_cleanup_list
> > only when the delayed work is scheduled. The patch has no effect
> > if the delayed work is not used.
> >
> > From my POV, this patch kind of removes the effect of the delayed
> > work. My point is:
> >
> > Does it still make sense to use the delayed work in the first place?
> > Will the delayed work still help to catch some problems?
>
> That depends on the user of CONFIG_DEBUG_KOBJECT_RELEASE, if users
> thought it is useless, I think it is fine to remove it.
>
> Greg, any idea about if CONFIG_DEBUG_KOBJECT_RELEASE is useful now?
Yes it is, it finds driver bugs where they do things wrong.
I've been ignoring this thread until after 5.16-rc1 is out, sorry.
greg k-h
next prev parent reply other threads:[~2021-11-10 7:03 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-11-05 6:37 [PATCH 0/2] kobject: avoid to cleanup kobject after module is unloaded Ming Lei
2021-11-05 6:37 ` [PATCH 1/2] kobject: don't delay to cleanup module kobject Ming Lei
2021-11-26 16:08 ` Greg Kroah-Hartman
2021-11-26 16:28 ` Ming Lei
2021-11-26 16:33 ` Greg Kroah-Hartman
2021-11-29 2:38 ` Ming Lei
2021-11-05 6:37 ` [PATCH 2/2] kobject: wait until kobject is cleaned up before freeing module Ming Lei
2021-11-05 14:10 ` kernel test robot
2021-11-05 14:54 ` Ming Lei
2021-11-08 17:26 ` Petr Mladek
2021-11-09 2:00 ` Ming Lei
2021-11-09 13:14 ` Petr Mladek
2021-11-10 1:20 ` Ming Lei
2021-11-10 7:03 ` Greg Kroah-Hartman [this message]
2021-11-10 9:05 ` Petr Mladek
2021-11-10 9:19 ` Greg Kroah-Hartman
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