Re: [bdi_unregister] 165a5e22fa INFO: task swapper:1 blocked for more than 120 seconds.

[James Bottomley <jejb@linux.vnet.ibm.com>]

On Mon, 2017-03-06 at 16:14 +0100, Jan Kara wrote:
> On Mon 06-03-17 06:35:21, James Bottomley wrote:
> > On Mon, 2017-03-06 at 13:01 +0100, Jan Kara wrote:
> > > On Mon 06-03-17 11:27:33, Jan Kara wrote:
> > > > Hi,
> > > > 
> > > > On Sun 05-03-17 10:21:11, Wu Fengguang wrote:
> > > > > FYI next-20170303 is good while mainline is bad with this
> > > > > error.
> > > > > The attached reproduce-* may help reproduce the issue.
> > > > 
> > > > Thanks for report! So from the stacktrace we are in the path 
> > > > testing removal of a device immediately after it has been
> > > > probed 
> > > > and for some reason bdi_unregister() hangs - likely waiting for
> > > > cgroup-writeback references to drop. Given how early this
> > > > happens 
> > > > my guess is we fail to initialize something but for now I don't
> > > > see 
> > > > how my patch could make a difference. I'm trying to reproduce
> > > > this 
> > > > to be able to debug more...
> > > 
> > > OK, so after some debugging I think this is yet another problem
> > > in 
> > > SCSI initialization / destruction code which my patch only makes 
> > > visible (added relevant maintainers).
> > > 
> > > I can reproduce the problem reliably with enabling:
> > > 
> > > CONFIG_DEBUG_TEST_DRIVER_REMOVE=y
> > > CONFIG_SCSI_DEBUG=m
> > > CONFIG_BLK_CGROUP=y
> > > CONFIG_MEMCG=y
> > > (and thus CONFIG_CGROUP_WRITEBACK=y)
> > > 
> > > then 'modprobe scsi_debug' is all it takes to reproduce hang. 
> > > Relevant kernel messages with some of my debugging added
> > > (attached is 
> > > a patch that adds those debug messages):
> > 
> > This looks to be precisely the same problem Dan Williams was
> > debugging
> > for us.
> > 
> > > [   58.721765] scsi host0: scsi_debug: version 1.86 [20160430]
> > > [   58.721765]   dev_size_mb=8, opts=0x0, submit_queues=1,
> > > statistics=0
> > > [   58.728946] CGWB init ffff88007fbb2000
> > > [   58.730095] Created sdev ffff880078e1a000
> > > [   58.731611] scsi 0:0:0:0: Direct-Access     Linux   
> > >  scsi_debug
> > > 0186 PQ : 0 ANSI: 7
> > > [   58.782246] sd 0:0:0:0: [sda] 16384 512-byte logical blocks:
> > > (8.39
> > > MB/8.00 MiB)
> > > [   58.789687] sd 0:0:0:0: [sda] Write Protect is off
> > > [   58.791140] sd 0:0:0:0: [sda] Mode Sense: 73 00 10 08
> > > [   58.800879] sd 0:0:0:0: [sda] Write cache: enabled, read
> > > cache:
> > > enabled, supports DPO and FUA
> > > [   58.893738] sd 0:0:0:0: [sda] Attached SCSI disk
> > > [   58.896808] Unreg1
> > > [   58.897960] Unreg2
> > > [   58.898637] Unreg3
> > > [   58.899100] CGWB ffff88007fbb2000 usage_cnt: 0
> > > [   58.900004] Unreg4
> > > [   58.904976] sd 0:0:0:0: [sda] Synchronizing SCSI cache
> > 
> > OK, can you put a WARN_ON trace in sd_shutdown and tell us where
> > this
> > is coming from.  For the device to be reused after this we have to
> > be
> > calling sd_shutdown() without going into SDEV_DEL.
> 
> Sure. The call trace is:
> 
> [   41.919244] ------------[ cut here ]------------
> [   41.919263] WARNING: CPU: 4 PID: 2335 at drivers/scsi/sd.c:3332
> sd_shutdown+0x2f/0x100
> [   41.919268] Modules linked in: scsi_debug(+) netconsole loop btrfs
> raid6_pq zlib_deflate lzo_compress xor
> [   41.919319] CPU: 4 PID: 2335 Comm: modprobe Not tainted 4.11.0-rc1
> -xen+ #49
> [   41.919325] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
> [   41.919331] Call Trace:
> [   41.919343]  dump_stack+0x8e/0xf0
> [   41.919354]  __warn+0x116/0x120
> [   41.919361]  warn_slowpath_null+0x1d/0x20
> [   41.919368]  sd_shutdown+0x2f/0x100
> [   41.919374]  sd_remove+0x70/0xd0
> 
> *** Here is the unexpected step I guess...
> 
> [   41.919383]  driver_probe_device+0xe0/0x4c0

Exactly.  It's this, I think

	bool test_remove = IS_ENABLED(CONFIG_DEBUG_TEST_DRIVER_REMOVE) &&
			   !drv->suppress_bind_attrs;

You have that config option set.

So the drivers base layer is calling ->remove after probe and
triggering the destruction of the queue.

What to do about this (apart from nuke such a stupid option) is
somewhat more problematic.

James