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

Lai Jiangshan
2019-11-18T01:54:36Z

On 2019/11/18 5:53 上午, Paul E. McKenney wrote:
> On Sat, Nov 16, 2019 at 09:04:56PM +0800, Lai Jiangshan wrote:
>> 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.
>
> It is quite possible that special.b.deferred_qs might be useful
> for debugging. But it should now be possible to take care of the
> nohz_full issue for expedited grace periods, which might in turn allow
> rcu_read_unlock_special() to avoid acquiring scheduler locks.
>
> This could avoid the need for negative ->rcu_read_lock_nesting,
> in turn allowing your simplified _rcu_read_unlock().
>
> Would you like to do the expedited grace-period modifications, or
> would you rather that I do so?
>

Hello, Paul

To be honest, I didn't known there was special issue about
nohz_full with expedited grace periods until several days before
you told me. I just thought that it is requested to be expedited
so that we need to wake up something to handle it ASAP.

IOW, I'm not in a position to do the expedited grace-period
modifications before I learnt enough about it. I would be very
obliged that you do so. I believe it will be a better solution
than this one or the one in V2 relying on preempt_count.

Thanks
Lai