linux-kernel - Re: mips qemu test failures in -next due to "kthread: Fix use-after-free if kthread fork fails"

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Date:   Sun, 28 May 2017 07:44:30 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Vegard Nossum <vegard.nossum@...cle.com>
Cc:     Oleg Nesterov <oleg@...hat.com>,
        Jamie Iles <jamie.iles@...cle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: mips qemu test failures in -next due to "kthread: Fix
 use-after-free if kthread fork fails"

+cc Linus, as mainline is now affected.

On 05/28/2017 04:45 AM, Vegard Nossum wrote:
> On 05/27/17 19:56, Guenter Roeck wrote:
>> Hi,
>>
>> my qemu testis of mips images are failing in -next. Symptom is a hang during
>> boot; see http://kerneltests.org/builders/qemu-mips-next for some examples.
>>
>> I bisected the problem in next-20170526. It points to commit 4d6501dce079c
>> ("kthread: Fix use-after-free if kthread fork fails"). Reverting that patch
>> fixes the problem.
>>
>> Bisect log is attached.
> 
> Hi,
> 
> Thanks for the report and sorry for the breakage :-/
> 
> I can't immediately spot what's going wrong, but I am able to reproduce
> it on mips so I will try to debug.
> 
> Are you sure it's this commit, though? I checked out linus/master and
> I get a boot hang even after reverting it.
> 

Reverting the patch fixed the problem for me in -next. The last mainline build
on my test system was with v4.12-rc2-228-gc86daad, which was fine except for
the known build breakages (frv, hexagon) and the known tracebacks
in crisv32
	WARNING: CPU: 0 PID: 1 at drivers/mtd/mtdcore.c:734 mtd_device_parse_register+0x17a/0x1aa
and ppc
	refcount_t: underflow; use-after-free.
	WARNING: CPU: 0 PID: 1 at lib/refcount.c:184 refcount_sub_and_test+0x84/0xe0

It may well be that something else is broken in mainline. The build of
v4.12-rc2-284-g249f1ef on my builders isn't complete yet. We'll see in
a couple of hours if there is other damage. I'll bisect later today if
necessary.

Guenter