rcu: Permit rt_mutex_unlock() with irqs disabled

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.

This version removes a redundant local_irq_restore() noted by
Yong Zhang.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

2 files changed, 13 insertions, 0 deletions

diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index d3127e8764cb..28422767d854 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
@@ -1211,6 +1213,9 @@ 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. */
diff --git a/kernel/rtmutex.c b/kernel/rtmutex.c
index 255e1662acdb..5e8d9cce7470 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);
         }