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Subject: Re: frequent lockups in 3.18rc4
From: Linus Torvalds <torvalds@linux-foundation.org>
To: Dave Jones <davej@codemonkey.org.uk>,
        Linus Torvalds <torvalds@linux-foundation.org>,
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        Sasha Levin <sasha.levin@oracle.com>,
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On Sun, Dec 21, 2014 at 4:41 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> The second time (or third, or fourth - it might not take immediately)
> you get a lockup or similar. Bad things happen.

I've only tested it twice now, but the first time I got a weird
lockup-like thing (things *kind* of worked, but I could imagine that
one CPU was stuck with a lock held, because things eventually ground
to a screeching halt.

The second time I got

  INFO: rcu_sched self-detected stall on CPU { 5}  (t=84533 jiffies
g=11971 c=11970 q=17)

and then

   INFO: rcu_sched detected stalls on CPUs/tasks: { 1 2 3 4 5 6 7}
(detected by 0, t=291309 jiffies, g=12031, c=12030, q=57)

with backtraces that made no sense (because obviously no actual stall
had taken place), and were the CPU's mostly being idle.

I could easily see it resulting in your softlockup scenario too.

                          Linus