linux-kernel - Re: fs/dcache.c - BUG: soft lockup - CPU#5 stuck for 22s! [systemd-udevd:1667]

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Date:	Wed, 28 May 2014 10:37:51 +0300
From:	Mika Westerberg <mika.westerberg@...ux.intel.com>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Miklos Szeredi <mszeredi@...e.cz>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: fs/dcache.c - BUG: soft lockup - CPU#5 stuck for 22s!
 [systemd-udevd:1667]

On Wed, May 28, 2014 at 04:19:55AM +0100, Al Viro wrote:
> On Tue, May 27, 2014 at 10:04:09AM +0300, Mika Westerberg wrote:
> > On Tue, May 27, 2014 at 05:00:26AM +0100, Al Viro wrote:
> > > On Tue, May 27, 2014 at 04:14:15AM +0100, Al Viro wrote:
> > > 
> > > > As the matter of fact, let's try this instead - retry the same sucker
> > > > immediately in case if trylocks fail.  Comments?
> > > 
> > > Better yet, let's take "move back to shrink list" into dentry_kill()
> > > itself.  Then we get consistent locking rules for dentry_kill() and
> > > instead of unlock_on_failure we simply pass it NULL or the shrink
> > > list to put the sucker back.  Mika, could you test this one and see
> > > if it fixes that livelock?  The difference in behaviour is that in
> > > case of trylock failure we hit that sucker again without letting
> > > it ride all the way around the list, same as we do for other dentry_kill()
> > > callers.
> > 
> > I tried this patch and unfortunately it still results the same sort of
> > livelock. I've attached the dmesg.
> > 
> > I also tried the serialization patch from Linus and it seemed to fix the
> > problem. After several rounds of USB memory stick plug/unplug I haven't
> > seen a single "soft lockup" warning in dmesg.
> > 
> > I'm able to reproduce the problem pretty easily, so if you have
> > something else to try I'm more than happy to give it a try.
> 
> Could you try this and post the resulting log?  I'd really like to understand
> what's going on there - are we really hitting trylock failures there and what
> dentries are involved.

I sent you the whole log privately so that I don't spam everyone.

Summary is here:

May 28 10:24:23 lahna kernel: scsi 14:0:0:0: Direct-Access     JetFlash Transcend 4GB    8.07 PQ: 0 ANSI: 4
May 28 10:24:23 lahna kernel: sd 14:0:0:0: Attached scsi generic sg4 type 0
May 28 10:24:23 lahna kernel: sd 14:0:0:0: [sdc] 7864320 512-byte logical blocks: (4.02 GB/3.75 GiB)
May 28 10:24:23 lahna kernel: sd 14:0:0:0: [sdc] Write Protect is off
May 28 10:24:23 lahna kernel: sd 14:0:0:0: [sdc] Mode Sense: 23 00 00 00
May 28 10:24:23 lahna kernel: sd 14:0:0:0: [sdc] Write cache: disabled, read cache: enabled, doesn't support DPO or FUA
May 28 10:24:23 lahna kernel:  sdc: sdc1
May 28 10:24:23 lahna kernel: sd 14:0:0:0: [sdc] Attached SCSI removable disk

Here I detached the USB stick:

May 28 10:24:32 lahna kernel: usb 3-10.4: USB disconnect, device number 6
May 28 10:24:32 lahna kernel: check_submounts_and_drop[/dev/block/8:33]; CPU 4 PID 576 [systemd-udevd]
May 28 10:24:32 lahna kernel: dput[/dev/block/8:33]; CPU 4 PID 576 [systemd-udevd]
May 28 10:24:32 lahna kernel: check_submounts_and_drop[3-10/3-10.4/3-10.4:1.0/host14]; CPU 1 PID 1683 [systemd-udevd]
May 28 10:24:32 lahna kernel: shrink[3-10.4:1.0/host14/target14:0:0/subsystem]; CPU 1 PID 1683 [systemd-udevd]
May 28 10:24:32 lahna kernel: shrink[host14/target14:0:0/14:0:0:0/rev]; CPU 1 PID 1683 [systemd-udevd]
May 28 10:24:32 lahna kernel: shrink[host14/target14:0:0/14:0:0:0/vendor]; CPU 1 PID 1683 [systemd-udevd]
May 28 10:24:32 lahna kernel: shrink[host14/target14:0:0/14:0:0:0/model]; CPU 1 PID 1683 [systemd-udevd]
May 28 10:24:32 lahna kernel: check_submounts_and_drop[/dev/block/8:32]; CPU 4 PID 576 [systemd-udevd]
...
May 28 10:24:32 lahna kernel: check_submounts_and_drop[0000:00:14.0/usb3/3-10/3-10.4]; CPU 6 PID 1684 [systemd-udevd]
May 28 10:24:32 lahna kernel: check_submounts_and_drop[0000:00:14.0/usb3/3-10/3-10.4]; CPU 7 PID 1685 [systemd-udevd]
May 28 10:24:32 lahna kernel: shrink[usb3/3-10/3-10.4/ltm_capable]; CPU 7 PID 1685 [systemd-udevd]
May 28 10:24:32 lahna kernel: A
May 28 10:24:32 lahna kernel: shrink[usb3/3-10/3-10.4/ltm_capable]; CPU 7 PID 1685 [systemd-udevd]
May 28 10:24:32 lahna kernel: A
May 28 10:24:32 lahna kernel: shrink[usb3/3-10/3-10.4/ltm_capable]; CPU 7 PID 1685 [systemd-udevd]
May 28 10:24:32 lahna kernel: A
May 28 10:24:32 lahna kernel: shrink[usb3/3-10/3-10.4/ltm_capable]; CPU 7 PID 1685 [systemd-udevd]
May 28 10:24:32 lahna kernel: A
May 28 10:24:32 lahna kernel: shrink[usb3/3-10/3-10.4/ltm_capable]; CPU 7 PID 1685 [systemd-udevd]
May 28 10:24:32 lahna kernel: A
May 28 10:24:32 lahna kernel: shrink[usb3/3-10/3-10.4/ltm_capable]; CPU 7 PID 1685 [systemd-udevd]
May 28 10:24:32 lahna kernel: A
May 28 10:24:32 lahna kernel: shrink[usb3/3-10/3-10.4/ltm_capable]; CPU 7 PID 1685 [systemd-udevd]
May 28 10:24:32 lahna kernel: A
May 28 10:24:32 lahna kernel: shrink[usb3/3-10/3-10.4/ltm_capable]; CPU 7 PID 1685 [systemd-udevd]
May 28 10:24:32 lahna kernel: A
May 28 10:24:32 lahna kernel: shrink[usb3/3-10/3-10.4/ltm_capable]; CPU 7 PID 1685 [systemd-udevd]
May 28 10:24:32 lahna kernel: A

This goes on an on with a variable amount of 'A's printed.
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