On Thu, May 31, 2018 at 03:42:12PM +0200, Jan Kara wrote:

On Thu 31-05-18 22:19:44, Tetsuo Handa wrote:

quoted

On 2018/05/31 20:42, Jan Kara wrote:

quoted

On Thu 31-05-18 01:00:08, Tetsuo Handa wrote:

quoted

So, we have no idea what is happening...
Then, what about starting from temporary debug printk() patch shown below?

quoted

From 4f70f72ad3c9ae6ce1678024ef740aca4958e5b0 Mon Sep 17 00:00:00 2001

From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Date: Wed, 30 May 2018 09:57:10 +0900
Subject: [PATCH] bdi: Add temporary config for debugging wb_workfn() versus
 bdi_unregister() race bug.

syzbot is hitting NULL pointer dereference at wb_workfn() [1]. But due to
limitations that syzbot cannot find reproducer for this bug (frequency is
once or twice per a day) nor we can't capture vmcore in the environment
which syzbot is using, for now we need to rely on printk() debugging.

[1] https://syzkaller.appspot.com/bug?id=e0818ccb7e46190b3f1038b0c794299208ed4206

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>

Hum a bit ugly solution but if others are fine with this, I can live with
it for a while as well. Or would it be possible for syzkaller to just test
some git tree where this patch is included? Then we would not even have to
have the extra config option...

If syzbot can reproduce this bug that way. While it is possible to add/remove
git trees syzbot tests, frequently adding/removing trees is bothering.

syzbot can enable extra config option. Maybe the config name should be
something like CONFIG_DEBUG_FOR_SYZBOT rather than individual topic.

I think that syzbot is using many VM instances. I don't know how many
instances will be needed for reproducing this bug within reasonable period.
More git trees syzbot tests, (I assume that) longer period will be needed
for reproducing this bug. The most reliable way is to use the shared part
of all trees (i.e. linux.git).

I understand this, I'd be just a bit reluctant to merge temporary debug
patches like this to Linus' tree only to revert them later just because
syzkaller... What do others think?

Don't commit temporary debug crap to the mainline kernel. Enable
syzkaller to run against a user supplied git tree specification if
it needs special debug to track down a problem.

-Dave.
-- 
Dave Chinner
david@fromorbit.com