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Subject[PATCH tip/core/rcu 41/55] rcu: Permit rt_mutex_unlock() with irqs disabled
Date
From: Paul E. McKenney <paul.mckenney@linaro.org>

Create a separate lockdep class for the rt_mutex used for RCU priority
boosting and enable use of rt_mutex_lock() with irqs disabled. This
prevents RCU priority boosting from falling prey to deadlocks when
someone begins an RCU read-side critical section in preemptible state,
but releases it with an irq-disabled lock held.

Unfortunately, the scheduler's runqueue and priority-inheritance locks
still must either completely enclose or be completely enclosed by any
overlapping RCU read-side critical section.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
kernel/rcutree_plugin.h | 6 ++++++
kernel/rtmutex.c | 8 ++++++++
2 files changed, 14 insertions(+), 0 deletions(-)
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index d3127e8..f6c63ea 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -1149,6 +1149,8 @@ static void rcu_initiate_boost_trace(struct rcu_node *rnp)

#endif /* #else #ifdef CONFIG_RCU_TRACE */

+static struct lock_class_key rcu_boost_class;
+
/*
* Carry out RCU priority boosting on the task indicated by ->exp_tasks
* or ->boost_tasks, advancing the pointer to the next task in the
@@ -1211,10 +1213,14 @@ static int rcu_boost(struct rcu_node *rnp)
*/
t = container_of(tb, struct task_struct, rcu_node_entry);
rt_mutex_init_proxy_locked(&mtx, t);
+ /* Avoid lockdep false positives. This rt_mutex is its own thing. */
+ lockdep_set_class_and_name(&mtx.wait_lock, &rcu_boost_class,
+ "rcu_boost_mutex");
t->rcu_boost_mutex = &mtx;
raw_spin_unlock_irqrestore(&rnp->lock, flags);
rt_mutex_lock(&mtx); /* Side effect: boosts task t's priority. */
rt_mutex_unlock(&mtx); /* Keep lockdep happy. */
+ local_irq_restore(flags);

return rnp->exp_tasks != NULL || rnp->boost_tasks != NULL;
}
diff --git a/kernel/rtmutex.c b/kernel/rtmutex.c
index ab44911..2548f44 100644
--- a/kernel/rtmutex.c
+++ b/kernel/rtmutex.c
@@ -579,6 +579,7 @@ __rt_mutex_slowlock(struct rt_mutex *lock, int state,
struct rt_mutex_waiter *waiter)
{
int ret = 0;
+ int was_disabled;

for (;;) {
/* Try to acquire the lock: */
@@ -601,10 +602,17 @@ __rt_mutex_slowlock(struct rt_mutex *lock, int state,

raw_spin_unlock(&lock->wait_lock);

+ was_disabled = irqs_disabled();
+ if (was_disabled)
+ local_irq_enable();
+
debug_rt_mutex_print_deadlock(waiter);

schedule_rt_mutex(lock);

+ if (was_disabled)
+ local_irq_disable();
+
raw_spin_lock(&lock->wait_lock);
set_current_state(state);
}
--
1.7.3.2


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