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SubjectRe: frequent lockups in 3.18rc4
On Sun, Dec 21, 2014 at 04:52:28PM -0800, Linus Torvalds 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.

So something trinity does when it doesn't have a better idea of
something to pass a syscall is to generate a random number.

A wild hypothesis could be that we're in one of these situations,
and we randomly generated 0xfed000f0 and passed that as a value to
a syscall, and the kernel wrote 0 to that address.

What syscall could do that, and not just fail a access_ok() or similar
is a mystery though.

Dave



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