Re: Simplifying our RCU models

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Tue, Mar 06, 2018 at 09:47:38AM +0100, Ingo Molnar wrote:
> 
> * Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:
> 
> > > > But if we look at the bigger API picture:
> > > >
> > > >                           !PREEMPT_RCU  PREEMPT_RCU=y
> > > >   rcu_read_lock():        atomic        preemptible
> > > >   rcu_read_lock_sched():  atomic        atomic
> > > >   srcu_read_lock():       preemptible   preemptible
> > > >
> > > > Then we could maintain full read side API flexibility by making PREEMPT_RCU=y the 
> > > > only model, merging it with SRCU and using these main read side APIs:
> > > >
> > > >   rcu_read_lock_preempt_disable():	atomic
> > > >   rcu_read_lock():			preemptible
> > 
> > One issue with merging SRCU into rcu_read_lock() is the general blocking within 
> > SRCU readers.  Once merged in, these guys block everyone.  We should focus 
> > initially on the non-SRCU variants.
> > 
> > On the other hand, Linus's suggestion of merging rcu_read_lock_sched()
> > into rcu_read_lock() just might be feasible.  If that really does pan
> > out, we end up with the following:
> > 
> >				!PREEMPT	PREEMPT=y
> >	rcu_read_lock():	atomic		preemptible
> >	srcu_read_lock():	preemptible	preemptible
> > 
> > In this model, rcu_read_lock_sched() maps to preempt_disable() and (as
> > you say above) rcu_read_lock_bh() maps to local_bh_disable().  The way
> > this works is that in PREEMPT=y kernels, synchronize_rcu() waits not
> > only for RCU read-side critical sections, but also for regions of code
> > with preemption disabled.  The main caveat seems to be that there be an
> > assumed point of preemptibility between each interrupt and each softirq
> > handler, which should be OK.
> > 
> > There will be some adjustments required for lockdep-RCU, but that should
> > be reasonably straightforward.
> > 
> > Seem reasonable?
> 
> Yes, that approach sounds very reasonable to me: it is similar to what we do on 
> the locking side as well, where we have 'atomic' variants (spinlocks/rwlocks) and 
> 'sleeping' variants (mutexes, rwsems, etc.).
> 
> ( This means there will be more automatic coupling between BH and preempt critical
>   sections and RCU models not captured via explicit RCU-namespace APIs, but that
>   should be OK I think. )

Thus far, I have been unable to prove that it cannot work, which is about
as good as it gets at this stage.  So here is hoping!  ;-)

I will look at your later corrected message, but will gratefully accept
your offer of help with the naming transition.

							Thanx, Paul

> A couple of small side notes:
> 
>  - Could we please also clean up the namespace of the synchronization APIs and 
>    change them all to an rcu_ prefix, like all the other RCU APIs are? Right now 
>    have a mixture like rcu_read_lock() but synchronize_rcu(), while I'd reall love 
>    to be able to do:
> 
> 	git grep '\<rcu_' ...
> 
>    ... to see RCU API usage within a particular kernel area. This would also clean
>    up some of the internal inconsistencies like having 'struct rcu_synchronize'.
> 
>  - If we are cleaning up the write side APIs, could we move over to a _wait 
>    nomenclature, i.e. rcu_wait*()?
> 
>    I.e. the new RCU namespace would be something like:
> 
>      rcu_read_lock                => rcu_read_lock        # unchanged
>      rcu_read_unlock              => rcu_read_unlock      # unchanged
> 
>      call_rcu                     => rcu_call_rcu
>      call_rcu_bh                  => rcu_call_bh
>      call_rcu_sched               => rcu_call_sched
> 
>      synchronize_rcu              => rcu_wait_
>      synchronize_rcu_bh           => rcu_wait_bh
>      synchronize_rcu_bh_expedited => rcu_wait_expedited_bh
>      synchronize_rcu_expedited    => rcu_wait_expedited
>      synchronize_rcu_mult         => rcu_wait_mult
>      synchronize_rcu_sched        => rcu_wait_sched
>      synchronize_rcu_tasks        => rcu_wait_tasks
> 
>      srcu_read_lock               => srcu_read_lock       # unchanged
>      srcu_read_unlock             => srcu_read_unlock     # unchanged
> 
>      synchronize_srcu             => srcu_wait
>      synchronize_srcu_expedited   => srcu_wait_expedited
> 
>    Note that due to the prefix approach we gain various new patterns:
> 
>        git grep rcu_wait          # matches both rcu and srcu
>        git grep rcu_wait          # matches all RCU waiting variants
>        git grep wait_expedited    # matches all expedited variants
> 
>    ... which all increase the organization of the namespace.
> 
>  - While we are at it, the two RCU-state API variants, while rarely used, are
>    named in a pretty obscure, disconnected fashion as well. A much better naming 
>    would be:
> 
>      get_state_synchronize_rcu    => rcu_get_state
>      cond_synchronize_rcu         => rcu_wait_state
> 
>    ... or so. This would also move them into the new, unified rcu_ prefix 
>    namespace.
> 
> Note how consistent and hierarchical the new RCU API namespace is:
> 
> 	<subsystem-prefix>_<verb>[_<qualifier[s]>]
> 
> If you agree with the overall concept of this I'd be glad to help out with 
> scripting & testing the RCU namespace transition safely in an unintrusive fashion 
> once you've done the model unification work, with compatibility defines to not 
> create conflicts, churn and pain, etc.
> 
> Thanks,
> 
> 	Ingo
>