Re: [RFC] Deadlock via recursive wakeup via RCU with threadirqs

[Paul E. McKenney]

On Fri, Jun 28, 2019 at 03:24:07PM -0400, Joel Fernandes wrote:
> On Fri, Jun 28, 2019 at 11:52:19AM -0700, Paul E. McKenney wrote:
> > On Fri, Jun 28, 2019 at 08:40:26PM +0200, Sebastian Andrzej Siewior wrote:
> > > On 2019-06-28 08:30:50 [-0700], Paul E. McKenney wrote:
> > > > On Fri, Jun 28, 2019 at 03:54:33PM +0200, Peter Zijlstra wrote:
> > > > > On Thu, Jun 27, 2019 at 11:41:07AM -0700, Paul E. McKenney wrote:
> > > > > > Or just don't do the wakeup at all, if it comes to that.  I don't know
> > > > > > of any way to determine whether rcu_read_unlock() is being called from
> > > > > > the scheduler, but it has been some time since I asked Peter Zijlstra
> > > > > > about that.
> > > > > 
> > > > > There (still) is no 'in-scheduler' state.
> > > > 
> > > > Well, my TREE03 + threadirqs rcutorture test ran for ten hours last
> > > > night with no problems, so we just might be OK.
> > > > 
> > > > The apparent fix is below, though my approach would be to do backports
> > > > for the full set of related changes.
> > > > 
> > > > Joel, Sebastian, how goes any testing from your end?  Any reason
> > > > to believe that this does not represent a fix?  (Me, I am still
> > > > concerned about doing raise_softirq() from within a threaded
> > > > interrupt, but am not seeing failures.)
> 
> Are you concerned also about a regular process context executing in the
> scheduler and using RCU, having this issue?
> (not anything with threaded or not threaded IRQs, but just a path in the
> scheduler that uses RCU).
> 
> I don't think Sebastian's lock up has to do with the fact that an interrupt
> is threaded or not, except that ksoftirqd is awakened in the case where
> threadirqs is passed.

In current -rcu, the checks should suffice in the absence of threaded
interrupts.  They might also suffice for threaded interrupts, but a more
direct approach would be better, hence the in_interrupt() patch.

> > > For some reason it does not trigger as good as it did yesterday.
> > 
> > I swear that I wasn't watching!!!  ;-)
> > 
> > But I do know that feeling.
> 
> :-)
> 
> > > Commit
> > > - 23634ebc1d946 ("rcu: Check for wakeup-safe conditions in
> > >    rcu_read_unlock_special()") does not trigger the bug within 94
> > >    attempts.
> > > 
> > > - 48d07c04b4cc1 ("rcu: Enable elimination of Tree-RCU softirq
> > >   processing") needed 12 attempts to trigger the bug.
> > 
> > That matches my belief that 23634ebc1d946 ("rcu: Check for wakeup-safe
> > conditions in rcu_read_unlock_special()") will at least greatly decrease
> > the probability of this bug occurring.
> 
> I was just typing a reply that I can't reproduce it with:
>   rcu: Check for wakeup-safe conditions in rcu_read_unlock_special()
> 
> I am trying to revert enough of this patch to see what would break things,
> however I think a better exercise might be to understand more what the patch
> does why it fixes things in the first place ;-) It is probably the
> deferred_qs thing.

The deferred_qs flag is part of it!  Looking forward to hearing what
you come up with as being the critical piece of this commit.

							Thanx, Paul