Re: [PATCH 08/11] rcu: don't use negative ->rcu_read_lock_nesting

["Paul E. McKenney" <paulmck@kernel.org>]

On Thu, Oct 31, 2019 at 10:08:03AM +0000, Lai Jiangshan wrote:
> Negative ->rcu_read_lock_nesting was introduced to prevent
> scheduler deadlock which was just prevented by deferred qs.
> So negative ->rcu_read_lock_nesting is useless now and
> rcu_read_unlock() can be simplified.
> 
> And negative ->rcu_read_lock_nesting is bug-prone,
> it is good to kill it.
> 
> Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
> ---
>  kernel/rcu/tree_exp.h    | 30 ++----------------------------
>  kernel/rcu/tree_plugin.h | 21 +++++----------------
>  2 files changed, 7 insertions(+), 44 deletions(-)
> 
> diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> index c0d06bce35ea..9dcbd2734620 100644
> --- a/kernel/rcu/tree_exp.h
> +++ b/kernel/rcu/tree_exp.h
> @@ -621,11 +621,11 @@ static void rcu_exp_handler(void *unused)
>  	 * report the quiescent state, otherwise defer.
>  	 */
>  	if (!t->rcu_read_lock_nesting) {
> +		rdp->exp_deferred_qs = true;
>  		if (!(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK)) ||
>  		    rcu_dynticks_curr_cpu_in_eqs()) {
> -			rcu_report_exp_rdp(rdp);
> +			rcu_preempt_deferred_qs(t);
>  		} else {
> -			rdp->exp_deferred_qs = true;
>  			set_tsk_need_resched(t);
>  			set_preempt_need_resched();
>  		}
> @@ -646,32 +646,6 @@ static void rcu_exp_handler(void *unused)
>  		WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, true);
>  		return;
>  	}
> -
> -	/*
> -	 * The final and least likely case is where the interrupted
> -	 * code was just about to or just finished exiting the RCU-preempt
> -	 * read-side critical section, and no, we can't tell which.
> -	 * So either way, set ->deferred_qs to flag later code that
> -	 * a quiescent state is required.
> -	 *
> -	 * If the CPU is fully enabled (or if some buggy RCU-preempt
> -	 * read-side critical section is being used from idle), just
> -	 * invoke rcu_preempt_deferred_qs() to immediately report the
> -	 * quiescent state.  We cannot use rcu_read_unlock_special()
> -	 * because we are in an interrupt handler, which will cause that
> -	 * function to take an early exit without doing anything.
> -	 *
> -	 * Otherwise, force a context switch after the CPU enables everything.
> -	 */
> -	rdp->exp_deferred_qs = true;
> -	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);
> -		set_preempt_need_resched();
> -	}
>  }
>  
>  /* PREEMPTION=y, so no PREEMPTION=n expedited grace period to clean up after. */
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index dbded2b8c792..c62631c79463 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -344,8 +344,6 @@ static int rcu_preempt_blocked_readers_cgp(struct rcu_node *rnp)
>  }
>  
>  /* Bias and limit values for ->rcu_read_lock_nesting. */
> -#define RCU_NEST_BIAS INT_MAX
> -#define RCU_NEST_NMAX (-INT_MAX / 2)
>  #define RCU_NEST_PMAX (INT_MAX / 2)
>  
>  /*
> @@ -373,21 +371,15 @@ void __rcu_read_unlock(void)
>  {
>  	struct task_struct *t = current;
>  
> -	if (t->rcu_read_lock_nesting != 1) {
> -		--t->rcu_read_lock_nesting;
> -	} else {
> +	if (--t->rcu_read_lock_nesting == 0) {
>  		barrier();  /* critical section before exit code. */
> -		t->rcu_read_lock_nesting = -RCU_NEST_BIAS;
> -		barrier();  /* assign before ->rcu_read_unlock_special load */

But if we take an interrupt here, and the interrupt handler contains
an RCU read-side critical section, don't we end up in the same hole
that resulted in this article when the corresponding rcu_read_unlock()
executes?  https://lwn.net/Articles/453002/

							Thanx, Paul

>  		if (unlikely(READ_ONCE(t->rcu_read_unlock_special.s)))
>  			rcu_read_unlock_special(t);
> -		barrier();  /* ->rcu_read_unlock_special load before assign */
> -		t->rcu_read_lock_nesting = 0;
>  	}
>  	if (IS_ENABLED(CONFIG_PROVE_LOCKING)) {
>  		int rrln = t->rcu_read_lock_nesting;
>  
> -		WARN_ON_ONCE(rrln < 0 && rrln > RCU_NEST_NMAX);
> +		WARN_ON_ONCE(rrln < 0);
>  	}
>  }
>  EXPORT_SYMBOL_GPL(__rcu_read_unlock);
> @@ -535,12 +527,9 @@ 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) &&
> -		(!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);
> +	return (__this_cpu_read(rcu_data.exp_deferred_qs) ||
> +		READ_ONCE(t->rcu_read_unlock_special.s)) &&
> +	       !t->rcu_read_lock_nesting;
>  }
>  
>  /*
> -- 
> 2.20.1
>