Re: [PATCH RFC v2 08/15] bpf, x86: Maintain Tasks RCU trampoline nesting in the BPF trampoline

["Paul E. McKenney" <paulmck@xxxxxxxxxx>]

On Sat, Sep 12, 2026 at 12:40:55PM -0700, Alexei Starovoitov wrote:
> On Sat Sep 12, 2026 at 11:03 AM PDT, Paul E. McKenney wrote:
> >
> > In the old kernels, yes, we have current->trc_reader_nesting++.
> > In the newer kernels, Tasks Trace RCU is instead implemented in terms
> > of SRCU-fast, which instead increments per-CPU counters.  Which among
> > other thins is a bit faster and does not need to hook into the scheduler.
> 
> old kernels? I'm confused.
> rcu_read_lock_trace() in bpf-next is doing t->trc_reader_nesting++
> and then calls __srcu_read_lock_fast().
> 
> Are you talking about some RCU branch that you target for next merge window?

No, I was thinking of rcu_read_lock_tasks_trace(), forgetting that
rcu_read_lock_trace() is still used.  (For good reason, just be clear.)
Your comments are quite correct for rcu_read_lock_trace().

Hmmm...  Josep's using t->trc_reader_nesting would break for
partially overlapping RCU Tasks and rcu_read_lock_trace() readers.

But yes, your #5 makes sense:  Deprecate RCU Tasks, upgrade RCU Tasks
Trace to check for preemption from within trampolines, and move RCU
Tasks users over to the rcu_read_lock_trace() variant of RCU Tasks Trace.
(Or am I still missing your point?)

Josef, thoughts?

                                                        Thanx, Paul

> > So we have several ways forward:
> >
> > 1.  Revert the implementation of RCU Tasks Trace in terms of
> >     SRCU-fast, and use the existing current->trc_reader_nesting++,
> >     as you suggest.
> >
> > 2.  Deprecate RCU Tasks Trace entirely in favor of RCU Tasks
> >     augmented by rcu_tasks_trampoline_enter() and friends, as
> >     I was suggesting.
> >
> > 3.  Implement rcu_tasks_trampoline_enter() in terms of SRCU-fast,
> >     keeping the speedup, and put a synchronize_srcu() in the
> >     RCU Tasks grace-period mechanism.  This again deprecates
> >     RCU Tasks Trace entirely in favor of the augmented RCU Tasks.
> >
> > 4.  It is always good to explicitly state the apparent status quo,
> >     which involves redundant trampoline entry/exit overhead.
> >
> > 5.  As always, your additional ideas here!
> >
> >                                                     Thanx, Paul
>