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

Paul E. McKenney
2019-10-31T13:47:34Z

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@linux.alibaba.com>
> ---
> 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
>