Re: [PATCH 02/11] rcu: fix bug when rcu_exp_handler() in nested interrupt

[Paul E. McKenney]

On Thu, Oct 31, 2019 at 10:07:57AM +0000, Lai Jiangshan wrote:
> These is a possible bug (although which I can't triger yet)
> since 2015 8203d6d0ee78
> (rcu: Use single-stage IPI algorithm for RCU expedited grace period)
> 
>  rcu_read_unlock()
>   ->rcu_read_lock_nesting = -RCU_NEST_BIAS;
>   interrupt(); // before or after rcu_read_unlock_special()
>    rcu_read_lock()
>     fetch some rcu protected pointers
>     // exp GP starts in other cpu.
>     some works
>     NESTED interrupt for rcu_exp_handler();
>       report exp qs! BUG!

Why would a quiescent state for the expedited grace period be reported
here?  This CPU is still in an RCU read-side critical section, isn't it?

							Thanx, Paul

>     // exp GP completes and pointers are freed in other cpu
>     some works with the pointers. BUG
>    rcu_read_unlock();
>   ->rcu_read_lock_nesting = 0;
> 
> Although rcu_sched_clock_irq() can be in nested interrupt,
> there is no such similar bug since special.b.need_qs
> can only be set when ->rcu_read_lock_nesting > 0
> 
> Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
> ---
>  kernel/rcu/tree_exp.h    | 5 +++--
>  kernel/rcu/tree_plugin.h | 9 ++++++---
>  2 files changed, 9 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> index 6dec21909b30..c0d06bce35ea 100644
> --- a/kernel/rcu/tree_exp.h
> +++ b/kernel/rcu/tree_exp.h
> @@ -664,8 +664,9 @@ static void rcu_exp_handler(void *unused)
>  	 * Otherwise, force a context switch after the CPU enables everything.
>  	 */
>  	rdp->exp_deferred_qs = true;
> -	if (!(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK)) ||
> -	    WARN_ON_ONCE(rcu_dynticks_curr_cpu_in_eqs())) {
> +	if (rcu_preempt_need_deferred_qs(t) &&
> +	    (!(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK)) ||
> +	    WARN_ON_ONCE(rcu_dynticks_curr_cpu_in_eqs()))) {
>  		rcu_preempt_deferred_qs(t);
>  	} else {
>  		set_tsk_need_resched(t);
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index d4c482490589..59ef10da1e39 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -549,9 +549,12 @@ rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags)
>   */
>  static bool rcu_preempt_need_deferred_qs(struct task_struct *t)
>  {
> -	return (__this_cpu_read(rcu_data.exp_deferred_qs) ||
> -		READ_ONCE(t->rcu_read_unlock_special.s)) &&
> -	       t->rcu_read_lock_nesting <= 0;
> +	return (__this_cpu_read(rcu_data.exp_deferred_qs) &&
> +		(!t->rcu_read_lock_nesting ||
> +		 t->rcu_read_lock_nesting == -RCU_NEST_BIAS))
> +		||
> +		(READ_ONCE(t->rcu_read_unlock_special.s) &&
> +		 t->rcu_read_lock_nesting <= 0);
>  }
>  
>  /*
> -- 
> 2.20.1
>