Re: frequent lockups in 3.18rc4

From: Linus Torvalds
Date: Wed Nov 19 2014 - 14:38:16 EST

On Wed, Nov 19, 2014 at 11:15 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> I suspect that the regression was triggered by the seccomp pull, since
> that reworked a lot of this code.

Note that it turns out that Dave can apparently see the same problems
with 3.17, so it's not actually a regression. So it may have been
going on for a while.

> Just to make sure I understand: it says "NMI watchdog", but this trace
> is from a timer interrupt, not NMI, right?

Yeah. The kernel/watchdog.c code always says "NMI watchdog", but it's
actually just a regular tiemr function: watchdog_timer_fn() started
with hrtimer_start().

> Is it possible that we've managed to return to userspace with
> interrupts off somehow? A loop in userspace that somehow has
> interrupts off can cause all kinds of fun lockups.

That sounds unlikely, but if there is some stack corruption going on.

However, it wouldn't even explain things, because even if interrupts
had been disabled in user space, and even if that popf got executed,
this wouldn't be where they got enabled. That would be the :"sti" in
the system call entry path (hidden behind the ENABLE_INTERRUPTS

Of course, maybe Dave has paravirtualization enabled (what a crock
_that_ is), and there is something wrong with that whole code.

> I don't understand the logic of what enables TIF_NOHZ.

Yeah, that makes two of us. But..

> In 3.17, I don't think that code would run with context tracking on,
> although I don't immediately see any bugs here.

See above: the problem apparently isn't new. Although it is possible
that we have two different issues going on..

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