linux-kernel - Re: __blkg_lookup oops with 4.2-rcX

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Date:	Sat, 5 Sep 2015 14:38:01 -0400
From:	Tejun Heo <tj@...nel.org>
To:	"Richard W.M. Jones" <rjones@...hat.com>
Cc:	Josh Boyer <jwboyer@...oraproject.org>,
	Jeff Moyer <jmoyer@...hat.com>, msnitzer@...hat.com,
	Li Zefan <lizefan@...wei.com>,
	Johannes Weiner <hannes@...xchg.org>, cgroups@...r.kernel.org,
	"Linux-Kernel@...r. Kernel. Org" <linux-kernel@...r.kernel.org>
Subject: Re: __blkg_lookup oops with 4.2-rcX

On Sat, Sep 05, 2015 at 04:48:40PM +0100, Richard W.M. Jones wrote:
> On Sat, Sep 05, 2015 at 04:34:39PM +0100, Richard W.M. Jones wrote:
> > [   52.259269] BUG: unable to handle kernel NULL pointer dereference at 00000000000009c8
> > [   52.259269] IP: [<ffffffff813f8b10>] __blkg_lookup+0x40/0xe0
> 
> And also:
> 
> $ addr2line -e /usr/lib/debug/lib/modules/4.3.0-0.rc0.git7.1.rwmj3.fc24.x86_64/vmlinux ffffffff813f8b10
> /usr/src/debug/kernel-4.2.fc24/linux-4.3.0-0.rc0.git7.1.rwmj3.fc24.x86_64/block/blk-cgroup.c:158
> 
>     152         /*
>     153          * Hint didn't match.  Look up from the radix tree.  Note that the
>     154          * hint can only be updated under queue_lock as otherwise @blkg
>     155          * could have already been removed from blkg_tree.  The caller is
>     156          * responsible for grabbing queue_lock if @update_hint.
>     157          */
>     158         blkg = radix_tree_lookup(&blkcg->blkg_tree, q->id);

Okay, found the bug.  It was an existing problem which got exposed by
the recent bypass change.  Will soon post a patch.

Thanks a lot!

-- 
tejun
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