Re: [PATCH RFC v3 06/13] bpf: Take a Tasks Trace reader in the trampoline glue

[Alexei Starovoitov]

On Thu Sep 17, 2026 at 1:16 AM UTC, Josef Bacik wrote:
> On Wed, 16 Sep 2026 03:45:16 +0000, Alexei Starovoitov wrote:
> > On Tue Sep 15, 2026 at 1:17 PM UTC, Josef Bacik wrote:
> > >  	__acquires(RCU)
> > >  {
> > > +	bpf_tramp_read_lock_trace();
> > >  	rcu_read_lock_dont_migrate();
> >
> > This is double increment. rcu_read_lock_dont_migrate() includes
> > rcu_read_lock_trace().
>
> Unless I'm looking at the wrong tree it doesn't, on Linus' master and on
> bpf-next it is
>
> 	static __always_inline void rcu_read_lock_dont_migrate(void)
> 	{
> 		if (IS_ENABLED(CONFIG_PREEMPT_RCU))
> 			migrate_disable();
> 		rcu_read_lock();
> 	}
>
> so plain RCU plus migrate_disable(), no Tasks Trace reader. That is why
> the non-sleepable glue needs one added here: on these architectures the
> trampoline image the glue returns into is only kept alive by Tasks RCU
> while the task is a rcu_read_lock_trace() reader, and rcu_read_lock()
> does not give us that.

Right. I got confused. Since rcu_read_lock_trace() CS will cover
both sleepable and non-sleepable prog types let's do it once
per fentry+fmod_ret region and 2nd time for fexit region.

We probably don't want to hold it for the whole trampoline,
since orig_call will delay freeing of progs.