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

[Paul E. McKenney]

On Sun, Aug 18, 2019 at 06:35:11PM -0400, Joel Fernandes wrote:
> On Sun, Aug 18, 2019 at 06:32:30PM -0400, Joel Fernandes wrote:
> > On Sun, Aug 18, 2019 at 03:12:10PM -0700, Paul E. McKenney wrote:
> > > On Sun, Aug 18, 2019 at 05:49:48PM -0400, Joel Fernandes (Google) wrote:
> > > > When we're in hard interrupt context in rcu_read_unlock_special(), we
> > > > can still benefit from invoke_rcu_core() doing wake ups of rcuc
> > > > threads when the !use_softirq parameter is passed.  This is safe
> > > > to do so because:
> > > > 
> > > > 1. We avoid the scheduler deadlock issues thanks to the deferred_qs bit
> > > > introduced in commit 23634ebc1d94 ("rcu: Check for wakeup-safe
> > > > conditions in rcu_read_unlock_special()") by checking for the same in
> > > > this patch.
> > > > 
> > > > 2. in_irq() implies in_interrupt() which implies raising softirq will
> > > > not do any wake ups.
> > > > 
> > > > The rcuc thread which is awakened will run when the interrupt returns.
> > > > 
> > > > We also honor 25102de ("rcu: Only do rcu_read_unlock_special() wakeups
> > > > if expedited") thus doing the rcuc awakening only when none of the
> > > > following are true:
> > > >   1. Critical section is blocking an expedited GP.
> > > >   2. A nohz_full CPU.
> > > > If neither of these cases are true (exp == false), then the "else" block
> > > > will run to do the irq_work stuff.
> > > > 
> > > > This commit is based on a partial revert of d143b3d1cd89 ("rcu: Simplify
> > > > rcu_read_unlock_special() deferred wakeups") with an additional in_irq()
> > > > check added.
> > > > 
> > > > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> > > 
> > > OK, I will bite...  If it is safe to wake up an rcuc kthread, why
> > > is it not safe to do raise_softirq()?
> > 
> > Because raise_softirq should not be done and/or doesn't do anything
> > if use_softirq == false. In fact, RCU_SOFTIRQ doesn't even existing if
> > use_softirq == false. The "else if" condition of this patch uses for
> > use_softirq.
> > 
> > Or, did I miss your point?

I am concerned that added "else if" condition might not be sufficient
to eliminate all possible cases of the caller holding a scheduler lock,
which could result in deadlock in the ensuing wakeup.  Might be me missing
something, but such deadlocks have been a recurring problem in the past.

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.  ;-)

							Thanx, Paul

> > > And from the nit department, looks like some whitespace damage on the
> > > comments.
> > 
> > I will fix all of these in the change log, it was just a quick RFC I sent
> > with the idea, tagged as RFC and not yet for merging. I should also remove
> > the comment about " in_irq() implies in_interrupt() which implies raising
> > softirq" from the changelog since this patch is only concerned with the rcuc
> > kthread.
> 
> Ah, I see you mean the comments on the code. Perhaps something went wrong
> when I did 'git revert' on the original patch, or some such. Anyway, please
> consider this as RFC-grade only. And hopefully I have been writing better
> change logs (really trying!!).
> 
> thanks,
> 
>  - Joel
>