linux-kernel - Re: [PATCH 1/2] drivers: core: Don't try to use a dead glue

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Date:   Wed, 11 Jul 2018 09:32:24 +1000
From:   Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Joel Stanley <joel@....id.au>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] drivers: core: Don't try to use a dead glue_dir

On Tue, 2018-07-10 at 16:55 +0200, Greg Kroah-Hartman wrote:
> On Tue, Jul 10, 2018 at 09:44:33AM +1000, Benjamin Herrenschmidt wrote:
> > On Sat, 2018-07-07 at 18:48 +0200, Greg Kroah-Hartman wrote:
> > > No, kobject_get() should never happen on a 0 refcount object.  That
> > > being said, the code does allow it, so if things are messed up, it will
> > > happen.  I think that change happened when the switch to refcount_t
> > > occured, before then we would WARN_ON() if that ever happened.  I should
> > > go fix that up, and restore that old behavior, so that syzbot starts
> > > complaining loudly when stuff like that hits.
> > > 
> > > So I hate using kobject_get_unless_zero(), and resisted ever adding it
> > > to the tree as it shows a bad locking/tree situation as you point out
> > > here.  But for some reason, the block developers seemed to insist they
> > > needed it, and so it is in the tree for them.  I don't want it to spread
> > > if at all possible, which makes me want to reject this patch as this
> > > should be "a case that can never be hit".
> > 
> > Except it can in that situation... at least unless you get my patch 2/2
> > (or the newer one I'm about to send that avoids adding a child counter
> > and uses the one in kernfs instead).
> 
> I like that fix, which should make this patch obsolete, right?

Yes, for that specific issue, but Linus seemed to think patch 1 was the
"right thing to do" regardless...

I suggest you read the backlog of thread if you are interested in the
ins and outs of his position, we had a rather extensive discussion on
this stuff.

Cheers,
Ben.