Re: [PATCH RFC v3 03/13] rcu-tasks: Add a Tasks RCU implementation for reader-marked trampolines

[Paul E. McKenney]

On Wed, Sep 16, 2026 at 02:40:20PM +0200, Frederic Weisbecker wrote:
> Le Tue, Sep 15, 2026 at 04:56:41PM -0700, Paul E. McKenney a écrit :
> > > Alternatively the approach could be generalized to vanilla RCU, it could be
> > > possible to define a .text.rcu_no_qs section within which code running is
> > > considered as an RCU reader (with a pause while on the explicit RCU tasks
> > > section). It would be forbidden to voluntary sleep inside
> > > and to put explicit preemption points (CONFIG_PROVE_RCU could report misuses).
> > > 
> > > Based on IP, RCU could consider those interrupted section as readers. This would
> > > require PREEMPT_RCU though.
> > > 
> > > And then synchronize_rcu() would do the 1, 2, 4 jobs.
> > 
> > If I am following correctly (ha!), sleepable BPF programs rule out use
> > of RCU in this manner.
> > 
> > But your point is nevertheless valid, in that SRCU could be used.
> > And because rcu_read_lock_trace() is a thin wrapper around SRCU-fast, we
> > *might* be able to instead use rcu_read_lock_tasks_trace(), which would
> > skip the task-struct increment and decrement, saving a few instructions.
> > Then, instead of waiting for each task's counter to go to zero, instead
> > just invoke synchronize_rcu_tasks_trace().
> > 
> > Which is pretty close to what Josef is proposing, just with the new RCU
> > Tasks Trace read-side primitives.  I think.  ;-)
> > 
> > This assumes that we do not need to flatten partially overlapping RCU
> > Tasks Trace readers into one big reader.
> > 
> > Or am I missing something here?
> 
> Yes I think that's what Josef does in this patchset. The problem is about
> handling the few instructions:
> 
> 1) between the begining of the trampoline and the call to rcu_read_lock_trace()
> 
> 2) between the call to rcu_read_unlock_trace() and the end of the trampoline
> 
> So what I'm proposing is to make those two parts implicit RCU read lock sections.
> 
> So the whole trampoline would be .text.rcu_no_qs:
> 
> .text.rcu_no_qs trampoline:
>   __________________________________________________________________________________________
>  |Few instructions 1 | rcu_read_lock_trace() .... rcu_read_unlock_trace | Few instructions 2|
>  ___________________________________________________________________________________________
> 
> Then when a tick fires, rcu_flavor_sched_clock_irq() discards the interrupted
> code as QS if the IP was within .text.rcu_no_qs _unless_ it is in the
> rcu_read_lock_trace. Both are easy and quick to verify.
> 
> Also preempt_schedule_irq() would make sure to verify the same condition and
> enqueue the task as a GP blocker if preempting inside "Few instructions 1"
> or "Few instructions 2".

Ah, OK, I might be following now.  ;-)

We also need both versions of rcu_exp_handler() to check the IP as well,
given that sooner or later someone is going to want trampoline removal
to go faster.  Or am I still missing a turn in here somewhere?

> And since RCU tasks already does a synchronize RCU before and after the scan,
> that's all we would have to do.

This is going to need some *serious* documentation.

Also, what would be a good way to add tests for this to rcutorture?
Designate some new rcutorture function as being in .text.rcu_no_qs and
add this as another type of rcutorture reader?  Or is there a better way?

							Thanx, Paul