From: Lai Jiangshan <laijs@linux.alibaba.com>
To: linux-kernel@vger.kernel.org
Cc: Lai Jiangshan <laijs@linux.alibaba.com>,
"Paul E. McKenney" <paulmck@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: [PATCH 07/11] rcu: set special.b.deferred_qs before wake_up()
Date: Thu, 31 Oct 2019 10:08:02 +0000 [thread overview]
Message-ID: <20191031100806.1326-8-laijs@linux.alibaba.com> (raw)
In-Reply-To: <20191031100806.1326-1-laijs@linux.alibaba.com>
The patch 10f39bb1b2c1
(rcu: protect __rcu_read_unlock() against scheduler-using irq handlers)
unveiled a kind of deadlock and resolved the deadlock problem by
avoiding the condition when ->rcu_read_lock_nesting is zero &&
->rcu_read_unlock_special is non-zero. To achieve it, the commit
used negative values for ->rcu_read_lock_nesting.
But now we have deferred_qs mechanism, we can defer qs rather
than persevere in reporting qs and deadlock. All we need is
setting special.b.deferred_qs before scheduler locks
such as wake_up() and leave the qs deferred and return.
After this change, rcu_read_unlock_special() is safe to be
called in any context, including nested in __rcu_read_unlock()
in interrupt.
This change is important to change ->rcu_read_lock_nesting
back to non-negative and further simplify the rcu_read_unlock().
Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
---
kernel/rcu/tree_plugin.h | 15 ++++++++++++---
1 file changed, 12 insertions(+), 3 deletions(-)
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index e612c77dc446..dbded2b8c792 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -591,6 +591,7 @@ static void rcu_read_unlock_special(struct task_struct *t)
irqs_were_disabled = irqs_disabled_flags(flags);
if (preempt_bh_were_disabled || irqs_were_disabled) {
bool exp;
+ bool deferred_qs = t->rcu_read_unlock_special.b.deferred_qs;
struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
struct rcu_node *rnp = rdp->mynode;
@@ -599,9 +600,18 @@ static void rcu_read_unlock_special(struct task_struct *t)
(rdp->grpmask & rnp->expmask) ||
tick_nohz_full_cpu(rdp->cpu);
// Need to defer quiescent state until everything is enabled.
+ // In some cases when in_interrupt() returns false,
+ // raise_softirq_irqoff() has to call wake_up(),
+ // and the !deferred_qs says that scheduler locks
+ // cannot be held, so the wakeup will be safe now.
+ // But this wake_up() may have RCU critical section nested
+ // in the scheduler locks and its rcu_read_unlock() would
+ // call rcu_read_unlock_special() and then wake_up()
+ // recursively and deadlock if deferred_qs is still false.
+ // To avoid it, deferred_qs has to be set beforehand.
+ t->rcu_read_unlock_special.b.deferred_qs = true;
if (irqs_were_disabled && use_softirq &&
- (in_interrupt() ||
- (exp && !t->rcu_read_unlock_special.b.deferred_qs))) {
+ (in_interrupt() || (exp && !deferred_qs))) {
// Using softirq, safe to awaken, and we get
// no help from enabling irqs, unlike bh/preempt.
raise_softirq_irqoff(RCU_SOFTIRQ);
@@ -620,7 +630,6 @@ static void rcu_read_unlock_special(struct task_struct *t)
irq_work_queue_on(&rdp->defer_qs_iw, rdp->cpu);
}
}
- t->rcu_read_unlock_special.b.deferred_qs = true;
local_irq_restore(flags);
return;
}
--
2.20.1
next prev parent reply other threads:[~2019-10-31 10:08 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
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 ` Lai Jiangshan [this message]
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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