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

Paul E. McKenney
2019-11-01T12:33:26Z

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
>