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

[Frederic Weisbecker]

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".

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

Thanks.

-- 
Frederic Weisbecker
SUSE Labs