Re: [GIT PULL rcu/next] RCU commits for 3.1

[Peter Zijlstra <peterz@infradead.org>]

On Mon, 2011-11-07 at 16:16 +0000, Stephane Eranian wrote:
> On Mon, Nov 7, 2011 at 3:15 PM, Peter Zijlstra <peterz@infradead.org> wrote:
> > So far nobody seems to have stated if this is an actual problem or just
> > shutting up lockdep-prove-rcu? I very much suspect the latter, in which
> > case I really utterly hate the patch because it adds instructions to
> > fast-paths just to kill a debug warning.
> >
> I think the core issue at stake here is not so much the cgroup disappearing.
> It cannot go away because it is ref counted (perf_events does the necessary
> css_get()/css_put()). But it is rather the task disappearing while we
> are operating
> on its state.
> 
> I don't think task (prev or next) can disappear while we execute
> perf_cgroup_sched_out()/perf_cgroup_sched_in() because we are in the context
> switch code.

Right.

> What remains is:
>   * update_cgrp_time_from_event()
>     alway operates on current task
> 
>   * perf_cgroup_set_timestamp()
> 
>        - perf_event_task_tick() -> cpu_ctx_sched_in() but in this case
> it is on the current task
>        - perf_event_task_sched_in() in context switch code so I assume
> it is safe
>        - __perf_event_enable() but it is called on current
> 
>   - perf_cgroup_switch()
>     * perf_cgroup_sched_in()/perf_cgroup_sched_out() -> context switch code
> 
>     * perf_cgroup_attach()
>       called from cgroup code. Does not appear to hold task_lock().
>       the routine already grabs the rcu_read_lock() but it that enough
> to guarantee the task cannot
>       vanish. I would hope so, otherwise I think the cgroup attach
> code has a problem.

yeah, task_struct is rcu-freed

> In summary, unless I am mistaken, it looks to me that we may not need
> those new rcu_read_lock()
> calls after all.
> 
> Does anyone have a different analysis? 

The only other problem I could see is that perf_cgroup_sched_{in,out}
can race against perf_cgroup_attach_task() and make the wrong decision.
But then perf_cgroup_attach will call perf_cgroup_switch() to fix that
up again.