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Date:   Tue, 24 Mar 2020 12:52:55 -0400
From:   Joel Fernandes <joel@joelfernandes.org>
To:     "Paul E. McKenney" <paulmck@kernel.org>
Cc:     Steven Rostedt <rostedt@goodmis.org>, rcu@vger.kernel.org,
        linux-kernel@vger.kernel.org, kernel-team@fb.com, mingo@kernel.org,
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        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        vpillai@digitalocean.com
Subject: Re: [PATCH RFC v2 tip/core/rcu 01/22] sched/core: Add function to
 sample state of locked-down task
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On Tue, Mar 24, 2020 at 08:48:22AM -0700, Paul E. McKenney wrote:
[..] 
> > 
> > > diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
> > > index 44edd0a..43991a4 100644
> > > --- a/kernel/rcu/tree.h
> > > +++ b/kernel/rcu/tree.h
> > > @@ -455,6 +455,8 @@ static void rcu_bind_gp_kthread(void);
> > >  static bool rcu_nohz_full_cpu(void);
> > >  static void rcu_dynticks_task_enter(void);
> > >  static void rcu_dynticks_task_exit(void);
> > > +static void rcu_dynticks_task_trace_enter(void);
> > > +static void rcu_dynticks_task_trace_exit(void);
> > >  
> > >  /* Forward declarations for tree_stall.h */
> > >  static void record_gp_stall_check_time(void);
> > > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> > > index 9355536..f4a344e 100644
> > > --- a/kernel/rcu/tree_plugin.h
> > > +++ b/kernel/rcu/tree_plugin.h
> > > @@ -2553,3 +2553,21 @@ static void rcu_dynticks_task_exit(void)
> > >  	WRITE_ONCE(current->rcu_tasks_idle_cpu, -1);
> > >  #endif /* #if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL) */
> > >  }
> > > +
> > > +/* Turn on heavyweight RCU tasks trace readers on idle/user entry. */
> > > +static void rcu_dynticks_task_trace_enter(void)
> > > +{
> > > +#ifdef CONFIG_TASKS_RCU_TRACE
> > > +	if (IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB))
> > > +		current->trc_reader_special.b.need_mb = true;
> > 
> > If this is every called from middle of a reader section (that is we
> > transition from IPI-mode to using heavier reader-sections), then is a memory
> > barrier needed here just to protect the reader section that already started?
> 
> That memory barrier is provided by the memory ordering in the callers
> of rcu_dynticks_task_trace_enter() and rcu_dynticks_task_trace_exit(),
> namely, those callers' atomic_add_return() invocations.  These barriers
> pair with the pair of smp_rmb() calls in rcu_dynticks_zero_in_eqs(),
> which is in turn invoked from the function formerly known as
> trc_inspect_reader_notrunning(), AKA trc_inspect_reader().
> 
> This same pair of smp_rmb() calls also pair with the conditional smp_mb()
> calls in rcu_read_lock_trace() and rcu_read_unlock_trace().
> 
> In your scenario, the calls in rcu_read_lock_trace() and
> rcu_read_unlock_trace() wouldn't happen, but in that case the ordering
> from atomic_add_return() would suffice.
> 
> Does that work?  Or is there an ordering bug in there somewhere?

Thanks for explaining. Could the following scenario cause a problem?

If we consider the litmus test:

{
int x = 1;
int *y = &x;
int z = 1;
}

P0(int *x, int *z, int **y)
{
	int *r0;
	int r1;

	dynticks_eqs_trace_enter();

	rcu_read_lock();
	r0 = rcu_dereference(*y);

	dynticks_eqs_trace_exit(); // cut-off reader's mb wings :)

	r1 = READ_ONCE(*r0); // Reordering of this beyond the unlock() is bad.
	rcu_read_unlock();
}

P1(int *x, int *z, int **y)
{
	rcu_assign_pointer(*y, z);
	synchronize_rcu();
	WRITE_ONCE(*x, 0);
}

exists (0:r0=x /\ 0:r1=0)

Then the following situation can happen?

	READER					UPDATER

						y = &z;

	eqs_enter(); // full-mb

	rcu_read_lock(); // full-mb
	// r0 = x;
						// GP-start
						// ..zero_in_eqs() notices eqs, no IPI
	eqs_exit(); // full-mb

	// actual r1 = *x but will reorder

	rcu_read_unlock(); // no-mb
						// GP-finish as notices nesting = 0
						x = 0;
	// reordered r1 = *x = 0;


Basically r0=x /\ r1=0 happened because r1=0. Or did I miss something that
prevents it?

thanks,

 - Joel


> > thanks,
> > 
> >  - Joel
> > 
> > 
> > > +#endif /* #ifdef CONFIG_TASKS_RCU_TRACE */
> > > +}
> > > +
> > > +/* Turn off heavyweight RCU tasks trace readers on idle/user exit. */
> > > +static void rcu_dynticks_task_trace_exit(void)
> > > +{
> > > +#ifdef CONFIG_TASKS_RCU_TRACE
> > > +	if (IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB))
> > > +		current->trc_reader_special.b.need_mb = false;
> > > +#endif /* #ifdef CONFIG_TASKS_RCU_TRACE */
> > > +}