Re: [PATCH V2 2/7] rcu: cleanup rcu_preempt_deferred_qs()

[Lai Jiangshan <laijs@linux.alibaba.com>]

On 2019/11/3 10:01 上午, Boqun Feng wrote:
> Hi Jiangshan,
> 
> 
> I haven't checked the correctness of this patch carefully, but..
> 
> 
> On Sat, Nov 02, 2019 at 12:45:54PM +0000, Lai Jiangshan wrote:
>> Don't need to set ->rcu_read_lock_nesting negative, irq-protected
>> rcu_preempt_deferred_qs_irqrestore() doesn't expect
>> ->rcu_read_lock_nesting to be negative to work, it even
>> doesn't access to ->rcu_read_lock_nesting any more.
> 
> rcu_preempt_deferred_qs_irqrestore() will report RCU qs, and may
> eventually call swake_up() or its friends to wake up, say, the gp
> kthread, and the wake up functions could go into the scheduler code
> path which might have RCU read-side critical section in it, IOW,
> accessing ->rcu_read_lock_nesting.

Sure, thank you for pointing it out.

I should rewrite the changelog in next round. Like this:

rcu: cleanup rcu_preempt_deferred_qs()

IRQ-protected rcu_preempt_deferred_qs_irqrestore() itself doesn't
expect ->rcu_read_lock_nesting to be negative to work.

There might be RCU read-side critical section in it (from wakeup()
or so), 1711d15bf5ef(rcu: Clear ->rcu_read_unlock_special only once)
will ensure that ->rcu_read_unlock_special is zero and these RCU
read-side critical sections will not call rcu_read_unlock_special().

Thanks
Lai

===
PS: Were 1711d15bf5ef(rcu: Clear ->rcu_read_unlock_special only once)
not applied earlier, it will be protected by previous patch (patch1)
in this series
"rcu: use preempt_count to test whether scheduler locks is held"
when rcu_read_unlock_special() is called.



> 
> Again, haven't checked closely, but this argument in the commit log
> seems untrue.
> 
> Regards,
> Boqun
> 
>>
>> It is true that NMI over rcu_preempt_deferred_qs_irqrestore()
>> may access to ->rcu_read_lock_nesting, but it is still safe
>> since rcu_read_unlock_special() can protect itself from NMI.
>>
>> Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
>> ---
>>   kernel/rcu/tree_plugin.h | 5 -----
>>   1 file changed, 5 deletions(-)
>>
>> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
>> index aba5896d67e3..2fab8be2061f 100644
>> --- a/kernel/rcu/tree_plugin.h
>> +++ b/kernel/rcu/tree_plugin.h
>> @@ -552,16 +552,11 @@ static bool rcu_preempt_need_deferred_qs(struct task_struct *t)
>>   static void rcu_preempt_deferred_qs(struct task_struct *t)
>>   {
>>   	unsigned long flags;
>> -	bool couldrecurse = t->rcu_read_lock_nesting >= 0;
>>   
>>   	if (!rcu_preempt_need_deferred_qs(t))
>>   		return;
>> -	if (couldrecurse)
>> -		t->rcu_read_lock_nesting -= RCU_NEST_BIAS;
>>   	local_irq_save(flags);
>>   	rcu_preempt_deferred_qs_irqrestore(t, flags);
>> -	if (couldrecurse)
>> -		t->rcu_read_lock_nesting += RCU_NEST_BIAS;
>>   }
>>   
>>   /*
>> -- 
>> 2.20.1
>>