Re: [RFC v2] rcu/tree: Try to invoke_rcu_core() if in_irq() during unlock

[Joel Fernandes <joel@joelfernandes.org>]

On Sun, Aug 18, 2019 at 09:41:43PM -0400, Joel Fernandes wrote:
> On Sun, Aug 18, 2019 at 06:21:53PM -0700, Paul E. McKenney wrote:
[snip]
> > > > Also, your commit log's point #2 is "in_irq() implies in_interrupt()
> > > > which implies raising softirq will not do any wake ups."  This mention
> > > > of softirq seems a bit odd, given that we are going to wake up a rcuc
> > > > kthread.  Of course, this did nothing to quell my suspicions.  ;-)
> > > 
> > > Yes, I should delete this #2 from the changelog since it is not very relevant
> > > (I feel now). My point with #2 was that even if were to raise a softirq
> > > (which we are not), a scheduler wakeup of ksoftirqd is impossible in this
> > > path anyway since in_irq() implies in_interrupt().
> > 
> > Please!  Could you also add a first-principles explanation of why
> > the added condition is immune from scheduler deadlocks?
> 
> Sure I can add an example in the change log, however I was thinking of this
> example which you mentioned:
> https://lore.kernel.org/lkml/20190627173831.GW26519@linux.ibm.com/
> 
> 	previous_reader()
> 	{
> 		rcu_read_lock();
> 		do_something(); /* Preemption happened here. */
> 		local_irq_disable(); /* Cannot be the scheduler! */
> 		do_something_else();
> 		rcu_read_unlock();  /* Must defer QS, task still queued. */
> 		do_some_other_thing();
> 		local_irq_enable();
> 	}
> 
> 	current_reader() /* QS from previous_reader() is still deferred. */
> 	{
> 		local_irq_disable();  /* Might be the scheduler. */
> 		do_whatever();
> 		rcu_read_lock();
> 		do_whatever_else();
> 		rcu_read_unlock();  /* Must still defer reporting QS. */
> 		do_whatever_comes_to_mind();
> 		local_irq_enable();
> 	}
> 
> One modification of the example could be, previous_reader() could also do:
> 	previous_reader()
> 	{
> 		rcu_read_lock();
> 		do_something_that_takes_really_long(); /* causes need_qs in
> 							  the unlock_special_union to be set */
> 		local_irq_disable(); /* Cannot be the scheduler! */
> 		do_something_else();
> 		rcu_read_unlock();  /* Must defer QS, task still queued. */
> 		do_some_other_thing();
> 		local_irq_enable();
> 	}

The point you were making in that thread being, current_reader() ->
rcu_read_unlock() -> rcu_read_unlock_special() would not do any wakeups
because previous_reader() sets the deferred_qs bit.

Anyway, I will add all of this into the changelog.

thanks,

 - Joel