Re: [PATCH tip/core/rcu 0/41] More RCU flavor consolidation cleanup for v4.21/v5.0

[Sasha Levin]

On Mon, Nov 12, 2018 at 08:01:37AM -0800, Paul E. McKenney wrote:
> On Mon, Nov 12, 2018 at 09:07:50AM -0500, Mathieu Desnoyers wrote:
> > ----- On Nov 11, 2018, at 2:41 PM, paulmck paulmck@xxxxxxxxxxxxx wrote:
> >
> > > Hello!
> > >
> > > This series does additional cleanup for the RCU flavor consolidation,
> > > focusing primarily on uses of old API members, for example, so that
> > > call_rcu_bh() becomes call_rcu().  There are also a few straggling
> > > internal-to-RCU cleanups.
> > >
> > > 1.	Remove unused rcu_state externs, courtesy of Joel Fernandes.
> > >
> > > 2.	Fix rcu_{node,data} comments about gp_seq_needed, courtesy of
> > >	Joel Fernandes.
> > >
> > > 3.	Eliminate synchronize_rcu_mult() and its sole caller.
> > >
> > > 4.	Consolidate the RCU update functions invoked by sync.c.
> > >
> > > 5-41.	Replace old flavorful RCU API calls with the corresponding
> > >	vanilla calls.
> >
> > Hi Paul,
> >
> > Just a heads up: we might want to spell out warnings in very big letters
> > for anyone trying to backport code using RCU from post-4.21 kernels
> > back to older kernels. I fear that newer code will build just fine
> > on older kernels, but will spectacularly fail in hard-to-debug ways at
> > runtime.
> >
> > Renaming synchronize_rcu() and call_rcu() to something that did not
> > exist in prior kernels would prevent that. It may not be as pretty
> > though.
>
> From v4.20 rather than v4.21, but yes.  Would it make sense to have Sasha
> automatically flag -stable candidates going back past that boundary that
> contain call_rcu(), synchronize_rcu(), etc.?  Adding Sasha on CC, and
> I might be able to touch base with him this week.

We had a similar issue recently with a vfs change
(https://patchwork.kernel.org/patch/10604339/) leading to potentially
the same results as described above, we took it as is to avoid these
issues in the future, though this is a much smaller change than what's
proposed here.

We can look into an good way to solve this. While I can alert on
post-4.20 stable tagged patches that touch rcu, do you really want to be
dealing with this for the next 10+ years? It'll also means each of those
patches will need a manual backport.

Let's talk at Plumbers :)

--
Thanks,
Sasha