Re: [PATCH tip/core/rcu 13/22] rcu: Fix grace-period hangs due to race with CPU offline

[Peter Zijlstra]

On Wed, Jun 27, 2018 at 10:13:34PM -0700, Paul E. McKenney wrote:
> On Wed, Jun 27, 2018 at 07:51:34PM +0200, Peter Zijlstra wrote:
> > On Wed, Jun 27, 2018 at 08:57:21AM -0700, Paul E. McKenney wrote:
> > > > Another variant, which simply skips the wakeup whever ran on an offline
> > > > CPU, relying on the wakeup from rcutree_migrate_callbacks() right after
> > > > the CPU really is dead.
> > > 
> > > Cute!  ;-)
> > > 
> > > And a much smaller change.
> > > 
> > > However, this means that if someone indirectly and erroneously causes
> > > rcu_report_qs_rsp() to be invoked from an offline CPU, the result is an
> > > intermittent and difficult-to-debug grace-period hang.  A lockdep splat
> > > whose stack trace directly implicates the culprit is much better.
> > 
> > How so? We do an unconditional wakeup right after finding the offline
> > cpu dead. There is only very limited code between offline being true and
> > the CPU reporting in dead.
> 
> I am thinking more generally than this particular patch.  People
> sometimes invoke things from places they shouldn't, for example, the
> situation leading to your patch that allows use of RCU much earlier in
> the CPU-online process.  It is nicer to get a splat in those situations
> than a silent hang.

The rcu_rnp_online_cpus() thing would catch that, right? The public RCU
API isn't that big, and should already complain afaict.