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>

diff --git a/include/linux/init_task.h b/include/linux/init_task.h
index 03b2748..77fc43f 100644 (file)
--- a/include/linux/init_task.h
+++ b/include/linux/init_task.h
@@ -111,7 +111,7 @@ extern struct group_info init_groups;
 #ifdef CONFIG_PREEMPT_RCU
 #define INIT_TASK_RCU_PREEMPT(tsk)                                     \
        .rcu_read_lock_nesting = 0,                                     \
-       .rcu_read_unlock_special = 0,                                   \
+       .rcu_read_unlock_special.s = 0,                                 \
        .rcu_node_entry = LIST_HEAD_INIT(tsk.rcu_node_entry),           \
        INIT_TASK_RCU_TREE_PREEMPT()
 #else

diff --git a/include/linux/sched.h b/include/linux/sched.h
index ec8b347..42888d7 100644 (file)
--- 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 */

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index e466b40..0981c0c 100644 (file)
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -155,9 +155,8 @@ EXPORT_SYMBOL_GPL(rcu_batches_completed);
  * not in a quiescent state.  There might be any number of tasks blocked
  * while in an RCU read-side critical section.
  *
- * Unlike the other rcu_*_qs() functions, callers to this function
- * must disable irqs in order to protect the assignment to
- * ->rcu_read_unlock_special.
+ * As with the other rcu_*_qs() functions, callers to this function
+ * must disable preemption.
  */
 static void rcu_preempt_qs(int cpu)
 {
@@ -166,7 +165,7 @@ static void rcu_preempt_qs(int cpu)
        if (rdp->passed_quiesce == 0)
                trace_rcu_grace_period(TPS("rcu_preempt"), rdp->gpnum, TPS("cpuqs"));
        rdp->passed_quiesce = 1;
-       current->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_NEED_QS;
+       current->rcu_read_unlock_special.b.need_qs = false;
 }
 
 /*
@@ -190,14 +189,14 @@ static void rcu_preempt_note_context_switch(int cpu)
        struct rcu_node *rnp;
 
        if (t->rcu_read_lock_nesting > 0 &&
-           (t->rcu_read_unlock_special & RCU_READ_UNLOCK_BLOCKED) == 0) {
+           !t->rcu_read_unlock_special.b.blocked) {
 
                /* Possibly blocking in an RCU read-side critical section. */
                rdp = per_cpu_ptr(rcu_preempt_state.rda, cpu);
                rnp = rdp->mynode;
                raw_spin_lock_irqsave(&rnp->lock, flags);
                smp_mb__after_unlock_lock();
-               t->rcu_read_unlock_special |= RCU_READ_UNLOCK_BLOCKED;
+               t->rcu_read_unlock_special.b.blocked = true;
                t->rcu_blocked_node = rnp;
 
                /*
@@ -239,7 +238,7 @@ static void rcu_preempt_note_context_switch(int cpu)
                                       : rnp->gpnum + 1);
                raw_spin_unlock_irqrestore(&rnp->lock, flags);
        } else if (t->rcu_read_lock_nesting < 0 &&
-                  t->rcu_read_unlock_special) {
+                  t->rcu_read_unlock_special.s) {
 
                /*
                 * Complete exit from RCU read-side critical section on
@@ -257,9 +256,7 @@ static void rcu_preempt_note_context_switch(int cpu)
         * grace period, then the fact that the task has been enqueued
         * means that we continue to block the current grace period.
         */
-       local_irq_save(flags);
        rcu_preempt_qs(cpu);
-       local_irq_restore(flags);
 }
 
 /*
@@ -340,7 +337,7 @@ void rcu_read_unlock_special(struct task_struct *t)
        bool drop_boost_mutex = false;
 #endif /* #ifdef CONFIG_RCU_BOOST */
        struct rcu_node *rnp;
-       int special;
+       union rcu_special special;
 
        /* NMI handlers cannot block and cannot safely manipulate state. */
        if (in_nmi())
@@ -350,12 +347,13 @@ void rcu_read_unlock_special(struct task_struct *t)
 
        /*
         * If RCU core is waiting for this CPU to exit critical section,
-        * let it know that we have done so.
+        * let it know that we have done so.  Because irqs are disabled,
+        * t->rcu_read_unlock_special cannot change.
         */
        special = t->rcu_read_unlock_special;
-       if (special & RCU_READ_UNLOCK_NEED_QS) {
+       if (special.b.need_qs) {
                rcu_preempt_qs(smp_processor_id());
-               if (!t->rcu_read_unlock_special) {
+               if (!t->rcu_read_unlock_special.s) {
                        local_irq_restore(flags);
                        return;
                }
@@ -368,8 +366,8 @@ void rcu_read_unlock_special(struct task_struct *t)
        }
 
        /* Clean up if blocked during RCU read-side critical section. */
-       if (special & RCU_READ_UNLOCK_BLOCKED) {
-               t->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_BLOCKED;
+       if (special.b.blocked) {
+               t->rcu_read_unlock_special.b.blocked = false;
 
                /*
                 * Remove this task from the list it blocked on.  The
@@ -658,7 +656,7 @@ static void rcu_preempt_check_callbacks(int cpu)
        }
        if (t->rcu_read_lock_nesting > 0 &&
            per_cpu(rcu_preempt_data, cpu).qs_pending)
-               t->rcu_read_unlock_special |= RCU_READ_UNLOCK_NEED_QS;
+               t->rcu_read_unlock_special.b.need_qs = true;
 }
 
 #ifdef CONFIG_RCU_BOOST
@@ -941,7 +939,7 @@ void exit_rcu(void)
                return;
        t->rcu_read_lock_nesting = 1;
        barrier();
-       t->rcu_read_unlock_special = RCU_READ_UNLOCK_BLOCKED;
+       t->rcu_read_unlock_special.b.blocked = true;
        __rcu_read_unlock();
 }
 

diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index 9487b48..6fb9115 100644 (file)
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -93,7 +93,7 @@ void __rcu_read_unlock(void)
                barrier();  /* critical section before exit code. */
                t->rcu_read_lock_nesting = INT_MIN;
                barrier();  /* assign before ->rcu_read_unlock_special load */
-               if (unlikely(ACCESS_ONCE(t->rcu_read_unlock_special)))
+               if (unlikely(ACCESS_ONCE(t->rcu_read_unlock_special.s)))
                        rcu_read_unlock_special(t);
                barrier();  /* ->rcu_read_unlock_special load before assign */
                t->rcu_read_lock_nesting = 0;