On Thu, 16 Oct 2014, Vince Weaver wrote:
> > It looks like the
> > else if (task->perf_event_ctxp[ctxn])
> > err = -EAGAIN;
>
> It is indeed stuck there, waiting for task->perf_event_ctxp[1] to get
> set to zero, which never happens.
>
OK, so with some more printk()s, it looks like somehow the parent thread
is trying to open a software event on itself.
task->perf_event_ctxp[1]
has a valid pointer, but the ctx it points to has a ctx->lock of 0.
So perf_lock_task_context() always returns NULL.
So in find_get_context()
we get stuck in an infinite retry loop, waiting forever for
either ctx->lock to go positive or for task->perf_event_ctxp[1]
to go NULL, neither of which is going to happen.
Now to find out why this could happen. Probably something to do with
crazy RCU magic :(
Vince
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