rcu: decrease rcu_report_exp_rnp coupling with scheduler

PREEMPT_RCU read-side critical sections blocking an expedited grace
period invoke rcu_report_exp_rnp().  When the last such critical section
has completed, rcu_report_exp_rnp() invokes the scheduler to wake up the
task that invoked synchronize_rcu_expedited() -- needlessly holding the
root rcu_node structure's lock while doing so, thus needlessly providing
a way for RCU and the scheduler to deadlock.

This commit therefore releases the root rcu_node structure's lock before
calling wake_up().

Reported-by: Ed Tomlinson <edt@aei.ca>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 4 insertions, 2 deletions

diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index 75113cb7c4f..6abef3cfcbc 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -695,9 +695,12 @@ static void rcu_report_exp_rnp(struct rcu_state *rsp, struct rcu_node *rnp)
 
         raw_spin_lock_irqsave(&rnp->lock, flags);
         for (;;) {
-                if (!sync_rcu_preempt_exp_done(rnp))
+                if (!sync_rcu_preempt_exp_done(rnp)) {
+                        raw_spin_unlock_irqrestore(&rnp->lock, flags);
                         break;
+                }
                 if (rnp->parent == NULL) {
+                        raw_spin_unlock_irqrestore(&rnp->lock, flags);
                         wake_up(&sync_rcu_preempt_exp_wq);
                         break;
                 }
@@ -707,7 +710,6 @@ static void rcu_report_exp_rnp(struct rcu_state *rsp, struct rcu_node *rnp)
                 raw_spin_lock(&rnp->lock); /* irqs already disabled */
                 rnp->expmask &= ~mask;
         }
-        raw_spin_unlock_irqrestore(&rnp->lock, flags);
 }
 
 /*