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

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Message-ID: <c105fb1ff12533b4980098ecd411089e0630b6bf.camel@kernel.crashing.org>
Date:   Tue, 10 Jul 2018 09:44:33 +1000
From:   Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     "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 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).

Ben.