From: Lai Jiangshan <laijs@linux.alibaba.com>
To: Boqun Feng <boqun.feng@gmail.com>,
"Paul E. McKenney" <paulmck@kernel.org>
Cc: 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 02/11] rcu: fix bug when rcu_exp_handler() in nested interrupt
Date: Fri, 1 Nov 2019 10:29:41 +0800 [thread overview]
Message-ID: <155e3e05-e0dc-26a7-c940-f86a819ffb2e@linux.alibaba.com> (raw)
In-Reply-To: <20191101001948.GA182@boqun-laptop.fareast.corp.microsoft.com>
On 2019/11/1 8:19 上午, Boqun Feng wrote:
> On Thu, Oct 31, 2019 at 11:52:58AM -0700, Paul E. McKenney wrote:
>> On Thu, Oct 31, 2019 at 11:14:23PM +0800, Lai Jiangshan wrote:
>>>
>>>
>>> On 2019/10/31 10:31 下午, Paul E. McKenney wrote:
>>>> On Thu, Oct 31, 2019 at 06:47:31AM -0700, 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();
>>>>
>>>> Also, which platforms support nested interrupts? Last I knew, this was
>>>> prohibited.
>>>>
>>>>>> 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?
>>>>
>>>> And I now see what you were getting at here. Yes, the current code
>>>> assumes that interrupt-disabled regions, like hardware interrupt
>>>> handlers, cannot be interrupted. But if interrupt-disabled regions such
>>>> as hardware interrupt handlers can be interrupted (as opposed to being
>>>> NMIed), wouldn't that break a whole lot of stuff all over the place in
>>>> the kernel? So that sounds like an arch bug to me.
>>>
>>> I don't know when I started always assuming hardware interrupt
>>> handler can be nested by (other) interrupt. I can't find any
>>> documents say Linux don't allow nested interrupt handler.
>>> Google search suggests the opposite.
>
> FWIW, there is a LWN article talking about we disallow interrupt nesting
> in *most* cases:
>
> https://lwn.net/Articles/380931/
Much thanks for the information!
>
> , that's unless a interrupt handler explicitly calls
> local_irq_enable_in_hardirq(), it remains irq disabled, which means no
> nesting interrupt allowed.
>
Even so the problem here will be fixed by patch7/8.
>
>>
>> The results I am seeing look to be talking about threaded interrupt
>> handlers, which indeed can be interrupted by hardware interrupts. As can
>> softirq handlers. But these are not examples of a hardware interrupt
>> handler being interrupted by another hardware interrupt. For that to
>> work reasonably, something like a system priority level is required,
>> as in the old DYNIX/ptx kernel, or, going even farther back, DEC's RT-11.
>>
>>> grep -rIni nested Documentation/memory-barriers.txt Documentation/x86/
>>> It still have some words about nested interrupt handler.
>>
>> Some hardware does not differentiate between interrupts and exceptions,
>> for example, an illegal-instruction trap within an interrupt handler
>> might look in some ways like a nested interrupt.
>>
>>> The whole patchset doesn't depend on this patch, and actually
>>> it is reverted later in the patchset. Dropping this patch
>>> can be an option for next round.
>>
>> Sounds like a plan!
>>
>> Thanx, Paul
>>
> [...]
>
next prev parent reply other threads:[~2019-11-01 2:29 UTC|newest]
Thread overview: 45+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-31 10:07 [PATCH 00/11] rcu: introduce percpu rcu_preempt_depth Lai Jiangshan
2019-10-31 10:07 ` [PATCH 01/11] rcu: avoid leaking exp_deferred_qs into next GP Lai Jiangshan
2019-10-31 13:43 ` Paul E. McKenney
2019-10-31 18:19 ` Lai Jiangshan
2019-10-31 19:00 ` Paul E. McKenney
2019-10-31 10:07 ` [PATCH 02/11] rcu: fix bug when rcu_exp_handler() in nested interrupt Lai Jiangshan
2019-10-31 13:47 ` Paul E. McKenney
2019-10-31 14:20 ` Lai Jiangshan
2019-10-31 14:31 ` Paul E. McKenney
2019-10-31 15:14 ` Lai Jiangshan
2019-10-31 18:52 ` Paul E. McKenney
2019-11-01 0:19 ` Boqun Feng
2019-11-01 2:29 ` Lai Jiangshan [this message]
2019-10-31 10:07 ` [PATCH 03/11] rcu: clean up rcu_preempt_deferred_qs_irqrestore() Lai Jiangshan
2019-10-31 13:52 ` Paul E. McKenney
2019-10-31 15:25 ` Lai Jiangshan
2019-10-31 18:57 ` Paul E. McKenney
2019-10-31 19:02 ` Paul E. McKenney
2019-10-31 10:07 ` [PATCH 04/11] rcu: cleanup rcu_preempt_deferred_qs() Lai Jiangshan
2019-10-31 14:10 ` Paul E. McKenney
2019-10-31 14:35 ` Lai Jiangshan
2019-10-31 15:07 ` Paul E. McKenney
2019-10-31 18:33 ` Lai Jiangshan
2019-10-31 22:45 ` Paul E. McKenney
2019-10-31 10:08 ` [PATCH 05/11] rcu: clean all rcu_read_unlock_special after report qs Lai Jiangshan
2019-11-01 11:54 ` Paul E. McKenney
2019-10-31 10:08 ` [PATCH 06/11] rcu: clear t->rcu_read_unlock_special in one go Lai Jiangshan
2019-11-01 12:10 ` Paul E. McKenney
2019-11-01 16:58 ` Paul E. McKenney
2019-10-31 10:08 ` [PATCH 07/11] rcu: set special.b.deferred_qs before wake_up() Lai Jiangshan
2019-10-31 10:08 ` [PATCH 08/11] rcu: don't use negative ->rcu_read_lock_nesting Lai Jiangshan
2019-11-01 12:33 ` Paul E. McKenney
2019-11-16 13:04 ` Lai Jiangshan
2019-11-17 21:53 ` Paul E. McKenney
2019-11-18 1:54 ` Lai Jiangshan
2019-11-18 14:57 ` Paul E. McKenney
2019-10-31 10:08 ` [PATCH 09/11] rcu: wrap usages of rcu_read_lock_nesting Lai Jiangshan
2019-10-31 10:08 ` [PATCH 10/11] rcu: clear the special.b.need_qs in rcu_note_context_switch() Lai Jiangshan
2019-10-31 10:08 ` [PATCH 11/11] x86,rcu: use percpu rcu_preempt_depth Lai Jiangshan
2019-11-01 12:58 ` Paul E. McKenney
2019-11-01 13:13 ` Peter Zijlstra
2019-11-01 14:30 ` Paul E. McKenney
2019-11-01 15:32 ` Lai Jiangshan
2019-11-01 16:21 ` Paul E. McKenney
2019-11-01 15:47 ` Lai Jiangshan
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=155e3e05-e0dc-26a7-c940-f86a819ffb2e@linux.alibaba.com \
--to=laijs@linux.alibaba.com \
--cc=boqun.feng@gmail.com \
--cc=jiangshanlai@gmail.com \
--cc=joel@joelfernandes.org \
--cc=josh@joshtriplett.org \
--cc=linux-kernel@vger.kernel.org \
--cc=mathieu.desnoyers@efficios.com \
--cc=paulmck@kernel.org \
--cc=rcu@vger.kernel.org \
--cc=rostedt@goodmis.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).