On Thu, May 31, 2018 at 03:42:12PM +0200, 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? 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@xxxxxxxxxxxxx
