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

[Paul E. McKenney]

On Fri, Jun 28, 2019 at 07:12:41PM -0400, Joel Fernandes wrote:
> On Fri, Jun 28, 2019 at 03:25:47PM -0700, Paul E. McKenney wrote:
> > On Fri, Jun 28, 2019 at 05:40:18PM -0400, Joel Fernandes wrote:
> > > Hi Paul,
> > > 
> > > On Fri, Jun 28, 2019 at 01:04:23PM -0700, Paul E. McKenney wrote:
> > > [snip]
> > > > > > > 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.
> > > 
> > > The new deferred_qs flag indeed saves the machine from the dead-lock.
> > > 
> > > If we don't want the deferred_qs, then the below patch also fixes the issue.
> > > However, I am more sure than not that it does not handle all cases (such as
> > > what if we previously had an expedited grace period IPI in a previous reader
> > > section and had to to defer processing. Then it seems a similar deadlock
> > > would present. But anyway, the below patch does fix it for me! It is based on
> > > your -rcu tree commit 23634ebc1d946f19eb112d4455c1d84948875e31 (rcu: Check
> > > for wakeup-safe conditions in rcu_read_unlock_special()).
> > 
> > The point here being that you rely on .b.blocked rather than
> > .b.deferred_qs.  Hmmm...  There are a number of places that check all
> > the bits via the .s leg of the rcu_special union.  The .s check in
> > rcu_preempt_need_deferred_qs() should be OK because it is conditioned
> > on t->rcu_read_lock_nesting of zero or negative.
> > Do rest of those also work out OK?
> > 
> > It would be nice to remove the flag, but doing so clearly needs careful
> > review and testing.
> 
> Agreed. I am planning to do an audit of this code within the next couple of
> weeks so I will be on the look out for any optimization opportunities related
> to this. Will let you know if this can work. For now I like your patch better
> because it is more conservative and doesn't cause any space overhead.

Fixing the bug in a maintainable manner is the priority, to be sure.
However, simplifications, assuming that they work, are very much worth
considering as well.

And Murphy says that there are still a number of bugs and optimization
opportunities.  ;-)

> If you'd like, please free to included my Tested-by on it:
> 
> Tested-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>

Will do, thank you!

> If you had a chance, could you also point to me any tests that show
> performance improvement with the irqwork patch, on the expedited GP usecase?
> I'd like to try it out as well. I guess rcuperf should have some?

As a first thing to try, I suggest running rcuperf with both readers and
writers, with only expedited grace periods, and with most (or maybe even
all) CPUs having nohz_full enabled.

							Thanx, Paul