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.

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

diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index c44729f3a763dac3a1e4adc6ef9a3a03c171111f..b8e902c2f9eae096ce464a8aa56082c1d5e77b43 100644 (file)
--- 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 ab449117aaf293e5135a58bc7b66b6cd8b1d669e..0222e349713d568a327865cf7e615833fcd5af45 100644 (file)
--- 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 +602,16 @@ __rt_mutex_slowlock(struct rt_mutex *lock, int state,
 
                raw_spin_unlock(&lock->wait_lock);
 
+               if (was_disabled = irqs_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);
        }