On 5/31/18 7:42 AM, Jan Kara wrote: > On Thu 31-05-18 22:19:44, Tetsuo Handa wrote: >> On 2018/05/31 20:42, Jan Kara wrote: >>> On Thu 31-05-18 01:00:08, Tetsuo Handa wrote: >>>> So, we have no idea what is happening... >>>> Then, what about starting from temporary debug printk() patch shown below? >>>> >>>> >From 4f70f72ad3c9ae6ce1678024ef740aca4958e5b0 Mon Sep 17 00:00:00 2001 >>>> From: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx> >>>> 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@xxxxxxxxxxxxxxxxxxx> >>> >>> 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? I guess I don't understand why having it in Linus's tree would make a difference to syzkaller? If there is a compelling reason why that absolutely has to be done, I don't think it should be accompanied by a special Kconfig option for it. It should just be on unconditionally, with the intent to remove it before release. -- Jens Axboe
