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    SubjectRe: [PATCH RFC v2 tip/core/rcu 01/22] sched/core: Add function to sample state of locked-down task
    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 */
    > > > +}

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