[Bug testsuite/23493] Test suite makes all CPU stuck forever on kernel 4.16.16 (Fedora 27)

["fche at redhat dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=23493

--- Comment #12 from Frank Ch. Eigler <fche at redhat dot com> ---
(In reply to Mark Wielaard from comment #9)
> It might assume the current pid actually exists? Maybe
> current is NULL at this point and we should check for that first?

current is nonzero (I think by definition), but
task_active_pid_ns(current) (called within find_get_pid->find_vpid)
looks like it was 0.

So when we get called for a schedule tracepoint, the invoking task
might just be a mostly-dead one which doesn't even have a pid any more.
I'm not sure why this should occur only under heavy load
(installcheck-parallel), vs. all the time.  Maybe task garbage collection
occurs more in the former case.

I'm tempted to put in a probe-prologue detection of this
(maybe via (current->flags & PF_EXITING)) and reject the probe
hit entirely.

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