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 16:11:36 +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 12:57:01PM +0100, Al Viro wrote:
> On Wed, May 28, 2014 at 10:37:51AM +0300, Mika Westerberg wrote:
> 
> > 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
> ...
> 
> Hmm...  As it is, we have d_walk() trying _very_ hard in those situations.
> Could you add that on top of the previous and see if livelock changes?
> 
> diff --git a/fs/dcache.c b/fs/dcache.c
> index 42ae01e..4ce58d3 100644
> --- a/fs/dcache.c
> +++ b/fs/dcache.c
> @@ -1209,7 +1209,7 @@ static enum d_walk_ret select_collect(void *_data, struct dentry *dentry)
>  	 * ensures forward progress). We'll be coming back to find
>  	 * the rest.
>  	 */
> -	if (!list_empty(&data->dispose))
> +	if (data->found)
>  		ret = need_resched() ? D_WALK_QUIT : D_WALK_NORETRY;
>  out:
>  	return ret;

I added this on top of your previus patch and unfortunately the livelock
is still there :-(

>From the previous log, it looks like udev is doing exactly same operation
(check_submounts_and_drop) on 4 of the CPUs in paraller:

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: check_submounts_and_drop[0000:00:14.0/usb3/3-10/3-10.4]; CPU 1 PID 1683 [systemd-udevd]
May 28 10:24:32 lahna kernel: check_submounts_and_drop[0000:00:14.0/usb3/3-10/3-10.4]; CPU 0 PID 1686 [systemd-udevd]

I wonder why is that.
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