linux-kernel - Re: [PATCH v2 08/21] rcu: Add noinstr-fast rcu_read_{,un}lock_tasks

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Message-ID: <5e9b7d89-fbd9-48f2-a538-a3aeaab5d9ec@paulmck-laptop>
Date: Thu, 2 Oct 2025 06:38:28 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: rcu@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
	Kernel Team <kernel-team@...a.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Andrii Nakryiko <andrii@...nel.org>,
	Alexei Starovoitov <ast@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH v2 08/21] rcu: Add noinstr-fast
 rcu_read_{,un}lock_tasks_trace() APIs

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 <paulmck@...nel.org> 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