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

Lai Jiangshan
2019-10-31T14:21:34Z

On 2019/10/31 9:47 下午, Paul E. McKenney wrote:
> 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

Remember, the ->rcu_read_lock_nesting is -RCU_NEST_BIAS + 1 now.
In for rcu_exp_handler(), it goes into this branch:

rdp->exp_deferred_qs = true;
if (!(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK)) ||
WARN_ON_ONCE(rcu_dynticks_curr_cpu_in_eqs())) {
rcu_preempt_deferred_qs(t);
} else {

and rcu_preempt_deferred_qs(t) report the QS no matter what
the value of ->rcu_read_lock_nesting is if it is negative,
in other words, "-RCU_NEST_BIAS + 1" is not different from
"-RCU_NEST_BIAS"

>
>> // 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
>>