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

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

On 2019/11/1 8:33 下午, Paul E. McKenney wrote:
> 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/
> 
> 

Hello, Paul

I'm replying the email of V1, which is relying on deferred_qs changes
in [PATCH 07/11] (V1).
([PATCH 04/11](V1) relies on it too as you pointed out)

I hope I can answer the question wrt https://lwn.net/Articles/453002/
maybe partially.

With the help of deferred_qs mechanism and the special.b.deferred_qs
bit, I HOPED rcu_read_unlock_special() can find if itself is
risking in scheduler locks via special.b.deferred_qs bit.

--t->rcu_read_lock_nesting;
//outmost rcu c.s, rcu_read_lock_nesting is 0. but special is not zero
INTERRUPT
  // the fallowing code will normally be in_interrupt()
  // or NOT in_interrupt() when wakeup_softirqd() in invoke_softirq()
  // or NOT in_interrupt() when preempt_shedule_irq()
  // or other cases I missed.
  scheduler_lock()
  rcu_read_lock()
  rcu_read_unlock()
   // special has been set but with no special.b.deferred_qs
   rcu_read_unlock_special()
    raise_softirq_irqoff()
     wake_up() when !in_interrupt() // dead lock

preempt_shedule_irq() is guaranteed to clear rcu_read_unlock_special
when rcu_read_lock_nesting = 0 before calling into scheduler locks.

But, at least, what caused my hope to be failed was the case
wakeup_softirqd() in invoke_softirq() (which was once protected by
softirq in about 2 years between ec433f0c5152 and facd8b80c67a).
I don't think it is hard to fix it if we keep using
special.b.deferred_qs as this V1 series.

Thanks
Lai