From: Lai Jiangshan <laijs@linux.alibaba.com>
To: 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: Thu, 31 Oct 2019 22:20:43 +0800 [thread overview]
Message-ID: <b39c0b81-8091-f1ab-c42e-455fbf112c23@linux.alibaba.com> (raw)
In-Reply-To: <20191031134731.GP20975@paulmck-ThinkPad-P72>
On 2019/10/31 9:47 下午, 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();
>> 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?
>
> Thanx, Paul
Remember, the ->rcu_read_lock_nesting is -RCU_NEST_BIAS + 1 now.
In for rcu_exp_handler(), it goes into this branch:
rdp->exp_deferred_qs = true;
if (!(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK)) ||
WARN_ON_ONCE(rcu_dynticks_curr_cpu_in_eqs())) {
rcu_preempt_deferred_qs(t);
} else {
and rcu_preempt_deferred_qs(t) report the QS no matter what
the value of ->rcu_read_lock_nesting is if it is negative,
in other words, "-RCU_NEST_BIAS + 1" is not different from
"-RCU_NEST_BIAS"
>
>> // exp GP completes and pointers are freed in other cpu
>> some works with the pointers. BUG
>> rcu_read_unlock();
>> ->rcu_read_lock_nesting = 0;
>>
>> Although rcu_sched_clock_irq() can be in nested interrupt,
>> there is no such similar bug since special.b.need_qs
>> can only be set when ->rcu_read_lock_nesting > 0
>>
>> Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
>> ---
>> kernel/rcu/tree_exp.h | 5 +++--
>> kernel/rcu/tree_plugin.h | 9 ++++++---
>> 2 files changed, 9 insertions(+), 5 deletions(-)
>>
>> diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
>> index 6dec21909b30..c0d06bce35ea 100644
>> --- a/kernel/rcu/tree_exp.h
>> +++ b/kernel/rcu/tree_exp.h
>> @@ -664,8 +664,9 @@ static void rcu_exp_handler(void *unused)
>> * Otherwise, force a context switch after the CPU enables everything.
>> */
>> rdp->exp_deferred_qs = true;
>> - if (!(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK)) ||
>> - WARN_ON_ONCE(rcu_dynticks_curr_cpu_in_eqs())) {
>> + 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);
>> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
>> index d4c482490589..59ef10da1e39 100644
>> --- a/kernel/rcu/tree_plugin.h
>> +++ b/kernel/rcu/tree_plugin.h
>> @@ -549,9 +549,12 @@ rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags)
>> */
>> static bool rcu_preempt_need_deferred_qs(struct task_struct *t)
>> {
>> - return (__this_cpu_read(rcu_data.exp_deferred_qs) ||
>> - READ_ONCE(t->rcu_read_unlock_special.s)) &&
>> - t->rcu_read_lock_nesting <= 0;
>> + return (__this_cpu_read(rcu_data.exp_deferred_qs) &&
>> + (!t->rcu_read_lock_nesting ||
>> + t->rcu_read_lock_nesting == -RCU_NEST_BIAS))
>> + ||
>> + (READ_ONCE(t->rcu_read_unlock_special.s) &&
>> + t->rcu_read_lock_nesting <= 0);
>> }
>>
>> /*
>> --
>> 2.20.1
>>
next prev parent reply other threads:[~2019-10-31 14:21 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 [this message]
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
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
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