rcu: Remove local_irq_disable() in rcu_preempt_note_context_switch()

The rcu_preempt_note_context_switch() function is on a scheduling fast
path, so it would be good to avoid disabling irqs.  The reason that irqs
are disabled is to synchronize process-level and irq-handler access to
the task_struct ->rcu_read_unlock_special bitmask.  This commit therefore
makes ->rcu_read_unlock_special instead be a union of bools with a short
allowing single-access checks in RCU's __rcu_read_unlock().  This results
in the process-level and irq-handler accesses being simple loads and
stores, so that irqs need no longer be disabled.  This commit therefore
removes the irq disabling from rcu_preempt_note_context_switch().

Reported-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 9 insertions, 7 deletions

diff --git a/include/linux/sched.h b/include/linux/sched.h
index ec8b34722bcc..42888d715fb1 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1212,6 +1212,13 @@ struct sched_dl_entity {
         struct hrtimer dl_timer;
 };
 
+union rcu_special {
+        struct {
+                bool blocked;
+                bool need_qs;
+        } b;
+        short s;
+};
 struct rcu_node;
 
 enum perf_event_task_context {
@@ -1264,7 +1271,7 @@ struct task_struct {
 
 #ifdef CONFIG_PREEMPT_RCU
         int rcu_read_lock_nesting;
-        char rcu_read_unlock_special;
+        union rcu_special rcu_read_unlock_special;
         struct list_head rcu_node_entry;
 #endif /* #ifdef CONFIG_PREEMPT_RCU */
 #ifdef CONFIG_TREE_PREEMPT_RCU
@@ -2005,16 +2012,11 @@ extern void task_clear_jobctl_trapping(struct task_struct *task);
 extern void task_clear_jobctl_pending(struct task_struct *task,
                                       unsigned int mask);
 
-#ifdef CONFIG_PREEMPT_RCU
-#define RCU_READ_UNLOCK_BLOCKED (1 << 0) /* blocked while in RCU read-side. */
-#define RCU_READ_UNLOCK_NEED_QS (1 << 1) /* RCU core needs CPU response. */
-#endif /* #ifdef CONFIG_PREEMPT_RCU */
-
 static inline void rcu_copy_process(struct task_struct *p)
 {
 #ifdef CONFIG_PREEMPT_RCU
         p->rcu_read_lock_nesting = 0;
-        p->rcu_read_unlock_special = 0;
+        p->rcu_read_unlock_special.s = 0;
         p->rcu_blocked_node = NULL;
         INIT_LIST_HEAD(&p->rcu_node_entry);
 #endif /* #ifdef CONFIG_PREEMPT_RCU */