Re: [PATCH RFC v2 tip/core/rcu 01/22] sched/core: Add function to sample state of locked-down task

Joel Fernandes
2020-03-24T16:53:00Z

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 */
> > > +}