From: Lai Jiangshan <laijs@linux.alibaba.com>
To: paulmck@kernel.org
Cc: Boqun Feng <boqun.feng@gmail.com>,
linux-kernel@vger.kernel.org,
Josh Triplett <josh@joshtriplett.org>,
Steven Rostedt <rostedt@goodmis.org>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
Lai Jiangshan <jiangshanlai@gmail.com>,
Joel Fernandes <joel@joelfernandes.org>,
rcu@vger.kernel.org
Subject: Re: [PATCH V2 2/7] rcu: cleanup rcu_preempt_deferred_qs()
Date: Tue, 5 Nov 2019 10:09:15 +0800 [thread overview]
Message-ID: <e820852f-87ca-f974-2245-99833205e270@linux.alibaba.com> (raw)
In-Reply-To: <20191104145539.GY20975@paulmck-ThinkPad-P72>
On 2019/11/4 10:55 下午, Paul E. McKenney wrote:
> On Sun, Nov 03, 2019 at 01:01:21PM +0800, Lai Jiangshan wrote:
>>
>>
>> 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.
>
> This one in -rcu, you mean?
>
> 5c5d9065e4eb ("rcu: Clear ->rcu_read_unlock_special only once")
Yes, but the commit ID is floating in the tree.
>
> Some adjustment was needed due to my not applying the earlier patches
> that assumed nested interrupts. Please let me know if further adjustments
> are needed.
I don't think the earlier patches are needed. If the possible? nested
interrupts described in my previous emails is an issue, the patch
"rcu: don't use negative ->rcu_read_lock_nesting" in this
series is enough to fixed it. If any adjustments needed for
this series, I will just put the adjustments the series.
Thanks
Lai
>
> Thanx, Paul
>
>>> 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
>>>>
next prev parent reply other threads:[~2019-11-05 2:09 UTC|newest]
Thread overview: 39+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-11-02 12:45 [PATCH V2 0/7] rcu: introduce percpu rcu_preempt_depth Lai Jiangshan
2019-11-02 12:45 ` [PATCH V2 1/7] rcu: use preempt_count to test whether scheduler locks is held Lai Jiangshan
2019-11-15 16:53 ` Paul E. McKenney
2020-02-19 3:31 ` Joel Fernandes
2020-02-19 3:59 ` Paul E. McKenney
2019-11-02 12:45 ` [PATCH V2 2/7] rcu: cleanup rcu_preempt_deferred_qs() Lai Jiangshan
2019-11-03 2:01 ` Boqun Feng
2019-11-03 5:01 ` Lai Jiangshan
2019-11-04 14:55 ` Paul E. McKenney
2019-11-05 2:09 ` Lai Jiangshan [this message]
2019-11-05 7:19 ` Paul E. McKenney
2019-11-11 14:32 ` Paul E. McKenney
2019-11-12 1:28 ` Lai Jiangshan
2020-02-17 23:23 ` Paul E. McKenney
2020-02-18 14:41 ` Steven Rostedt
2020-02-18 16:43 ` Paul E. McKenney
2019-11-15 16:55 ` Paul E. McKenney
2019-11-02 12:45 ` [PATCH V2 3/7] rcu: remove useless special.b.deferred_qs Lai Jiangshan
2020-02-17 23:23 ` Paul E. McKenney
2019-11-02 12:45 ` [PATCH V2 4/7] rcu: don't use negative ->rcu_read_lock_nesting Lai Jiangshan
2020-02-17 23:26 ` Paul E. McKenney
2019-11-02 12:45 ` [PATCH V2 5/7] rcu: wrap usages of rcu_read_lock_nesting Lai Jiangshan
2019-11-15 22:25 ` Paul E. McKenney
2019-11-02 12:45 ` [PATCH V2 6/7] rcu: clear the special.b.need_qs in rcu_note_context_switch() Lai Jiangshan
2019-11-16 15:46 ` Paul E. McKenney
2019-11-02 12:45 ` [PATCH V2 7/7] x86,rcu: use percpu rcu_preempt_depth Lai Jiangshan
2019-11-02 16:30 ` Borislav Petkov
2019-11-03 4:33 ` Lai Jiangshan
2019-11-04 9:25 ` Sebastian Andrzej Siewior
2019-11-04 11:41 ` Lai Jiangshan
2019-11-04 12:09 ` Sebastian Andrzej Siewior
2019-11-16 15:48 ` Paul E. McKenney
2019-11-18 2:02 ` Lai Jiangshan
2019-11-18 14:59 ` Paul E. McKenney
2019-11-19 1:59 ` Lai Jiangshan
2019-11-19 21:14 ` Paul E. McKenney
2019-11-20 2:47 ` Lai Jiangshan
2019-11-21 4:02 ` Paul E. McKenney
2019-11-02 15:05 ` [PATCH V2 0/7] rcu: introduce " Paul E. McKenney
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