Re: [PATCH] rcu: Report a quiescent state when it's exactly in the state

[Byungchul Park]

On 2018-05-13 ìì 2:26, Steven Rostedt wrote:
> On Sat, 12 May 2018 07:41:19 -0700
> "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
>
> > Don't get me wrong, this discussion was quite useful to me.  We probably
> > need to at least change the comments, and perhaps the code as well.  But
> > I agree that we need input from Peter and Steven to make much more forward
> > progress.
>
> It's the weekend so I skimmed more than read this thread, but I will
> just add this.
>
> The table Joel posted is interesting, and perhaps we should keep things
> consistent with that. But that said, with respect to task-RCU, as
> nothing on a trampoline should ever call cond_resched() (and perhaps I
> should add code in lockdep that verifies this), we just want a
> quiescent state that tells us that the task has left the trampoline. A
> cond_resched() should be one of those points that does.
>
> It really has nothing to do with scheduling or preemption. The issue is
> that if a task is on a trampoline and gets preempted, there's no
> knowing when it is off that trampoline where we can free it. We need to
> have places in the kernel that we know is a quiescent state to move
> task-RCU forward. cond_resched() seems to be one of them. schedule
> itself can not be, because it can be called from an interrupt preempting
> a task while it is on the trampoline.

Exactly. I think Steven explained how we should consider them exactly.

> -- Steve
--
Thanks,
Byungchul