Re: [lkp] [printk] 34578dc67f: EIP is at vprintk_emit+0x1ea/0x600

Sergey Senozhatsky
2016-02-24T12:52:23Z

Hello,

On (02/24/16 12:46), Petr Mladek wrote:
[..]
> > and you get the NMI watchdog softlockup because you have a whole bunch of
> >
> > "of_overlay_destroy: Could not find overlay #6"
> > "### dt-test ### of_unittest_destroy_tracked_overlays: overlay destroy failed for #6"
> >
> > messages to print. seems that somehitng just pushes them in a loop.
> > there are too many of them:
>
> This sounds like a reasonable explanation. It seems that
> of_unittest_destroy_tracked_overlays() really ended in an infinite
> loop.
>
> But I am still curious why the softlookup points to
>
> [ 33.497718] EIP is at vprintk_emit+0x1ea/0x600
>
> Also there is on the stack
>
> [ 33.497741] [<c068e712>] vprintk_default+0x32/0x40
> [ 33.497741] [<c068e712>] vprintk_default+0x32/0x40
> [ 33.497744] [<c06fdf6e>] printk+0x11/0x13
> [ 33.497744] [<c06fdf6e>] printk+0x11/0x13
> [ 33.497748] [<c0df5eec>] of_unittest_overlay+0x8d1/0x900
> [ 33.497748] [<c0df5eec>] of_unittest_overlay+0x8d1/0x900
> [ 33.497750] [<c0df6b1f>] of_unittest+0xc04/0xc2d
> [ 33.497750] [<c0df6b1f>] of_unittest+0xc04/0xc2d
>
> I would expect that the soft lookup happens in console_unlock()
> called with IRQs disabled. It seems to me that of_unittest_overlay()
> is called with IRQs enabled.

watchdog has two different parts: hrtimer part (via IRQ) checks the
touch_ts and wakeups the updater smpboot kthread; and updater kthread,
that updates touch_ts. to get a lockup you can just keep the preemption
disabled for 20+ seconds, so updater kthread will not update touch_ts.
the next hrtimer irq will detect lockup. and the .config we have is

CONFIG_PREEMPT_NONE=y
# CONFIG_PREEMPT_VOLUNTARY is not set
# CONFIG_PREEMPT is not set

that can add up to the issue.

(well, at least watchdog was working this way the last time I saw it).

> I want to be sure that the patch in printk() did not introduce
> a deadlock that is visible only under a high printk load.

sure. I did additional intensive tests today, and saw no issues.
schematically, something like this:

u64 start = local_lock() >> 31UL:

{ preempt_disable(), local_irq_save() }
while (1) {
u64 now = local_clock() >> 31UL;
if (now - start > TIMEOUT)
goto out;
pr_err(">>>>>\n");
}
out:
{ preempt_enable(), local_irq_restore() }

> I guess that the softlookup was not visible before the
> printk patch was applied. I wonder if the patch made something
> worse. It was supposed to improve things but...

I think the softlockup under the given conditions (endless printk and
no preemption) will show up regardless the patches being applied; but
I see you point, of course.

-ss