linux-kernel - Re: [PATCH 02/11] rcu: fix bug when rcu_exp

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Message-ID: <b39c0b81-8091-f1ab-c42e-455fbf112c23@linux.alibaba.com>
Date:   Thu, 31 Oct 2019 22:20:43 +0800
From:   Lai Jiangshan <laijs@...ux.alibaba.com>
To:     paulmck@...nel.org
Cc:     linux-kernel@...r.kernel.org,
        Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>, rcu@...r.kernel.org
Subject: Re: [PATCH 02/11] rcu: fix bug when rcu_exp_handler() in nested
 interrupt



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@...ux.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
>>