linux-kernel - Re: Kernel panic - not syncing: Fatal exception in interrupt (file_free

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Message-Id: <20170929160713.GX3521@linux.vnet.ibm.com>
Date:   Fri, 29 Sep 2017 09:07:13 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Kyle Sanderson <kyle.leet@...il.com>,
        Linux-Kernal <linux-kernel@...r.kernel.org>
Subject: Re: Kernel panic - not syncing: Fatal exception in interrupt
 (file_free_rcu+0x14)

On Thu, Sep 28, 2017 at 09:00:52PM -0700, Linus Torvalds wrote:
> On Thu, Sep 28, 2017 at 8:32 PM, Kyle Sanderson <kyle.leet@...il.com> wrote:
> > Not sure if the stack is crap or not, but this looks like an RCU crash?
> >
> > https://i.imgur.com/sBnNe1p.jpg
> 
> Hmm. Not the clearest picture, and the "Code:" line in particular is
> missing the interesting part, but at a guess it's taking a fault in
> put_cred(), which inlines to
> 
>         if (atomic_dec_and_test(&(cred)->usage))
>                 __put_cred(cred);
> 
> and I think it's that "cred" pointer that may be NULL, which makes
> "&(cred)->usage" be a NULL pointer too, and you get a page fault when
> it tries to decrement the usage count.
> 
> Now, it goes without saying that the cred pointer should never *be*
> NULL on a filp that is on the RCU freeing list, because we always
> initialize file->f_cred when we allocate a file to the current creds.
> 
> So there's something odd going on. Possibly entirely unrelated memory
> corruption.
> 
> Nothing obvious stands out, I think we'd need to see more of a pattern
> of the problem to see what is up.

Kyle, if this is reproducible, please build your kernel with
CONFIG_DEBUG_OBJECTS_RCU_HEAD=y.  This can detect the call_rcu()
equivalent of a double free, and these double frees have sometimes
resulted in output looking something like the .jpg along with the
NULL-pointer problem that Linus suspects.

							Thanx, Paul