On Wed, Oct 01, 2025 at 06:37:33PM -0700, Alexei Starovoitov wrote:
> On Wed, Oct 1, 2025 at 7:48 AM Paul E. McKenney <[email protected]> wrote:
> >
> > +static inline struct srcu_ctr __percpu *rcu_read_lock_tasks_trace(void)
> > +{
> > + struct srcu_ctr __percpu *ret =
> > __srcu_read_lock_fast(&rcu_tasks_trace_srcu_struct);
> > +
> > + rcu_try_lock_acquire(&rcu_tasks_trace_srcu_struct.dep_map);
> > + if (!IS_ENABLED(CONFIG_TASKS_TRACE_RCU_NO_MB))
> > + smp_mb(); // Provide ordering on noinstr-incomplete
> > architectures.
> > + return ret;
> > +}
>
> ...
>
> > @@ -50,14 +97,15 @@ static inline void rcu_read_lock_trace(void)
> > {
> > struct task_struct *t = current;
> >
> > + rcu_try_lock_acquire(&rcu_tasks_trace_srcu_struct.dep_map);
> > if (t->trc_reader_nesting++) {
> > // In case we interrupted a Tasks Trace RCU reader.
> > - rcu_try_lock_acquire(&rcu_tasks_trace_srcu_struct.dep_map);
> > return;
> > }
> > barrier(); // nesting before scp to protect against interrupt
> > handler.
> > - t->trc_reader_scp =
> > srcu_read_lock_fast(&rcu_tasks_trace_srcu_struct);
> > - smp_mb(); // Placeholder for more selective ordering
> > + t->trc_reader_scp =
> > __srcu_read_lock_fast(&rcu_tasks_trace_srcu_struct);
> > + if (!IS_ENABLED(CONFIG_TASKS_TRACE_RCU_NO_MB))
> > + smp_mb(); // Placeholder for more selective ordering
> > }
>
> Since srcu_fast() __percpu pointers must be incremented/decremented
> within the same task, should we expose "raw" rcu_read_lock_tasks_trace()
> at all?
> rcu_read_lock_trace() stashes that pointer within a task,
> so implementation guarantees that unlock will happen within the same task,
> while _tasks_trace() requires the user not to do stupid things.
>
> I guess it's fine to have both versions and the amount of copy paste
> seems justified, but I keep wondering.
> Especially since _tasks_trace() needs more work on bpf trampoline
> side to pass this pointer around from lock to unlock.
> We can add extra 8 bytes to struct bpf_tramp_run_ctx and save it there,
> but set/reset run_ctx operates on current anyway, so it's not clear
> which version will be faster. I suspect _trace() will be good enough.
> Especially since trc_reader_nesting is kinda an optimization.
The idea is to convert callers and get rid of rcu_read_lock_trace()
in favor of rcu_read_lock_tasks_trace(), the reason being the slow
task_struct access on x86. But if the extra storage is an issue for
some use cases, we can keep both. In that case, I would of course reduce
the copy-pasta in a future patch.
Thanx, Paul
