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@@ -130,12 +130,12 @@ static void rcu_preempt_qs(int cpu)
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* We have entered the scheduler, and the current task might soon be
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* context-switched away from. If this task is in an RCU read-side
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* critical section, we will no longer be able to rely on the CPU to
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- * record that fact, so we enqueue the task on the appropriate entry
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- * of the blocked_tasks[] array. The task will dequeue itself when
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- * it exits the outermost enclosing RCU read-side critical section.
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- * Therefore, the current grace period cannot be permitted to complete
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- * until the blocked_tasks[] entry indexed by the low-order bit of
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- * rnp->gpnum empties.
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+ * record that fact, so we enqueue the task on the blkd_tasks list.
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+ * The task will dequeue itself when it exits the outermost enclosing
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+ * RCU read-side critical section. Therefore, the current grace period
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+ * cannot be permitted to complete until the blkd_tasks list entries
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+ * predating the current grace period drain, in other words, until
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+ * rnp->gp_tasks becomes NULL.
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*
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* Caller must disable preemption.
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*/
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@@ -143,7 +143,6 @@ static void rcu_preempt_note_context_switch(int cpu)
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{
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struct task_struct *t = current;
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unsigned long flags;
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- int phase;
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struct rcu_data *rdp;
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struct rcu_node *rnp;
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@@ -165,15 +164,26 @@ static void rcu_preempt_note_context_switch(int cpu)
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* (i.e., this CPU has not yet passed through a quiescent
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* state for the current grace period), then as long
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* as that task remains queued, the current grace period
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- * cannot end.
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+ * cannot end. Note that there is some uncertainty as
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+ * to exactly when the current grace period started.
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+ * We take a conservative approach, which can result
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+ * in unnecessarily waiting on tasks that started very
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+ * slightly after the current grace period began. C'est
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+ * la vie!!!
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*
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* But first, note that the current CPU must still be
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* on line!
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*/
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WARN_ON_ONCE((rdp->grpmask & rnp->qsmaskinit) == 0);
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WARN_ON_ONCE(!list_empty(&t->rcu_node_entry));
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- phase = (rnp->gpnum + !(rnp->qsmask & rdp->grpmask)) & 0x1;
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- list_add(&t->rcu_node_entry, &rnp->blocked_tasks[phase]);
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+ if ((rnp->qsmask & rdp->grpmask) && rnp->gp_tasks != NULL) {
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+ list_add(&t->rcu_node_entry, rnp->gp_tasks->prev);
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+ rnp->gp_tasks = &t->rcu_node_entry;
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+ } else {
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+ list_add(&t->rcu_node_entry, &rnp->blkd_tasks);
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+ if (rnp->qsmask & rdp->grpmask)
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+ rnp->gp_tasks = &t->rcu_node_entry;
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+ }
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raw_spin_unlock_irqrestore(&rnp->lock, flags);
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}
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@@ -210,10 +220,7 @@ EXPORT_SYMBOL_GPL(__rcu_read_lock);
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*/
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static int rcu_preempted_readers(struct rcu_node *rnp)
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{
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- int phase = rnp->gpnum & 0x1;
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-
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- return !list_empty(&rnp->blocked_tasks[phase]) ||
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- !list_empty(&rnp->blocked_tasks[phase + 2]);
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+ return rnp->gp_tasks != NULL;
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}
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/*
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@@ -252,6 +259,21 @@ static void rcu_report_unblock_qs_rnp(struct rcu_node *rnp, unsigned long flags)
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rcu_report_qs_rnp(mask, &rcu_preempt_state, rnp_p, flags);
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}
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+/*
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+ * Advance a ->blkd_tasks-list pointer to the next entry, instead
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+ * returning NULL if at the end of the list.
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+ */
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+static struct list_head *rcu_next_node_entry(struct task_struct *t,
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+ struct rcu_node *rnp)
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+{
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+ struct list_head *np;
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+
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+ np = t->rcu_node_entry.next;
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+ if (np == &rnp->blkd_tasks)
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+ np = NULL;
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+ return np;
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+}
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+
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/*
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* Handle special cases during rcu_read_unlock(), such as needing to
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* notify RCU core processing or task having blocked during the RCU
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@@ -262,6 +284,7 @@ static void rcu_read_unlock_special(struct task_struct *t)
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int empty;
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int empty_exp;
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unsigned long flags;
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+ struct list_head *np;
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struct rcu_node *rnp;
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int special;
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@@ -305,7 +328,12 @@ static void rcu_read_unlock_special(struct task_struct *t)
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empty = !rcu_preempted_readers(rnp);
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empty_exp = !rcu_preempted_readers_exp(rnp);
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smp_mb(); /* ensure expedited fastpath sees end of RCU c-s. */
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+ np = rcu_next_node_entry(t, rnp);
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list_del_init(&t->rcu_node_entry);
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+ if (&t->rcu_node_entry == rnp->gp_tasks)
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+ rnp->gp_tasks = np;
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+ if (&t->rcu_node_entry == rnp->exp_tasks)
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+ rnp->exp_tasks = np;
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t->rcu_blocked_node = NULL;
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/*
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@@ -361,18 +389,16 @@ EXPORT_SYMBOL_GPL(__rcu_read_unlock);
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static void rcu_print_detail_task_stall_rnp(struct rcu_node *rnp)
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{
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unsigned long flags;
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- struct list_head *lp;
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- int phase;
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struct task_struct *t;
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- if (rcu_preempted_readers(rnp)) {
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- raw_spin_lock_irqsave(&rnp->lock, flags);
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- phase = rnp->gpnum & 0x1;
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- lp = &rnp->blocked_tasks[phase];
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- list_for_each_entry(t, lp, rcu_node_entry)
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- sched_show_task(t);
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- raw_spin_unlock_irqrestore(&rnp->lock, flags);
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- }
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+ if (!rcu_preempted_readers(rnp))
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+ return;
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+ raw_spin_lock_irqsave(&rnp->lock, flags);
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+ t = list_entry(rnp->gp_tasks,
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+ struct task_struct, rcu_node_entry);
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+ list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry)
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+ sched_show_task(t);
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+ raw_spin_unlock_irqrestore(&rnp->lock, flags);
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}
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/*
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@@ -402,16 +428,14 @@ static void rcu_print_detail_task_stall(struct rcu_state *rsp)
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*/
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static void rcu_print_task_stall(struct rcu_node *rnp)
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{
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- struct list_head *lp;
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- int phase;
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struct task_struct *t;
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- if (rcu_preempted_readers(rnp)) {
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- phase = rnp->gpnum & 0x1;
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- lp = &rnp->blocked_tasks[phase];
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- list_for_each_entry(t, lp, rcu_node_entry)
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- printk(" P%d", t->pid);
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- }
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+ if (!rcu_preempted_readers(rnp))
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+ return;
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+ t = list_entry(rnp->gp_tasks,
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+ struct task_struct, rcu_node_entry);
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+ list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry)
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+ printk(" P%d", t->pid);
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}
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/*
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@@ -430,10 +454,15 @@ static void rcu_preempt_stall_reset(void)
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* period that still has RCU readers blocked! This function must be
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* invoked -before- updating this rnp's ->gpnum, and the rnp's ->lock
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* must be held by the caller.
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+ *
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+ * Also, if there are blocked tasks on the list, they automatically
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+ * block the newly created grace period, so set up ->gp_tasks accordingly.
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*/
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static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
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{
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WARN_ON_ONCE(rcu_preempted_readers(rnp));
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+ if (!list_empty(&rnp->blkd_tasks))
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+ rnp->gp_tasks = rnp->blkd_tasks.next;
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WARN_ON_ONCE(rnp->qsmask);
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}
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@@ -457,45 +486,49 @@ static int rcu_preempt_offline_tasks(struct rcu_state *rsp,
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struct rcu_node *rnp,
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struct rcu_data *rdp)
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{
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- int i;
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struct list_head *lp;
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struct list_head *lp_root;
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int retval = 0;
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struct rcu_node *rnp_root = rcu_get_root(rsp);
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- struct task_struct *tp;
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+ struct task_struct *t;
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if (rnp == rnp_root) {
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WARN_ONCE(1, "Last CPU thought to be offlined?");
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return 0; /* Shouldn't happen: at least one CPU online. */
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}
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- WARN_ON_ONCE(rnp != rdp->mynode &&
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- (!list_empty(&rnp->blocked_tasks[0]) ||
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- !list_empty(&rnp->blocked_tasks[1]) ||
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- !list_empty(&rnp->blocked_tasks[2]) ||
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- !list_empty(&rnp->blocked_tasks[3])));
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+
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+ /* If we are on an internal node, complain bitterly. */
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+ WARN_ON_ONCE(rnp != rdp->mynode);
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/*
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- * Move tasks up to root rcu_node. Rely on the fact that the
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- * root rcu_node can be at most one ahead of the rest of the
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- * rcu_nodes in terms of gp_num value. This fact allows us to
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- * move the blocked_tasks[] array directly, element by element.
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+ * Move tasks up to root rcu_node. Don't try to get fancy for
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+ * this corner-case operation -- just put this node's tasks
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+ * at the head of the root node's list, and update the root node's
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+ * ->gp_tasks and ->exp_tasks pointers to those of this node's,
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+ * if non-NULL. This might result in waiting for more tasks than
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+ * absolutely necessary, but this is a good performance/complexity
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+ * tradeoff.
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*/
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if (rcu_preempted_readers(rnp))
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retval |= RCU_OFL_TASKS_NORM_GP;
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if (rcu_preempted_readers_exp(rnp))
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retval |= RCU_OFL_TASKS_EXP_GP;
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- for (i = 0; i < 4; i++) {
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- lp = &rnp->blocked_tasks[i];
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- lp_root = &rnp_root->blocked_tasks[i];
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- while (!list_empty(lp)) {
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- tp = list_entry(lp->next, typeof(*tp), rcu_node_entry);
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- raw_spin_lock(&rnp_root->lock); /* irqs already disabled */
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- list_del(&tp->rcu_node_entry);
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- tp->rcu_blocked_node = rnp_root;
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- list_add(&tp->rcu_node_entry, lp_root);
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- raw_spin_unlock(&rnp_root->lock); /* irqs remain disabled */
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- }
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+ lp = &rnp->blkd_tasks;
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+ lp_root = &rnp_root->blkd_tasks;
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+ while (!list_empty(lp)) {
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+ t = list_entry(lp->next, typeof(*t), rcu_node_entry);
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+ raw_spin_lock(&rnp_root->lock); /* irqs already disabled */
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+ list_del(&t->rcu_node_entry);
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+ t->rcu_blocked_node = rnp_root;
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+ list_add(&t->rcu_node_entry, lp_root);
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+ if (&t->rcu_node_entry == rnp->gp_tasks)
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+ rnp_root->gp_tasks = rnp->gp_tasks;
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+ if (&t->rcu_node_entry == rnp->exp_tasks)
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+ rnp_root->exp_tasks = rnp->exp_tasks;
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+ raw_spin_unlock(&rnp_root->lock); /* irqs still disabled */
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}
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+ rnp->gp_tasks = NULL;
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+ rnp->exp_tasks = NULL;
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return retval;
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}
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@@ -586,8 +619,7 @@ static DEFINE_MUTEX(sync_rcu_preempt_exp_mutex);
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*/
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static int rcu_preempted_readers_exp(struct rcu_node *rnp)
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{
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- return !list_empty(&rnp->blocked_tasks[2]) ||
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- !list_empty(&rnp->blocked_tasks[3]);
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+ return rnp->exp_tasks != NULL;
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}
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/*
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@@ -647,12 +679,13 @@ static void rcu_report_exp_rnp(struct rcu_state *rsp, struct rcu_node *rnp)
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static void
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sync_rcu_preempt_exp_init(struct rcu_state *rsp, struct rcu_node *rnp)
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{
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- int must_wait;
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+ int must_wait = 0;
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raw_spin_lock(&rnp->lock); /* irqs already disabled */
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- list_splice_init(&rnp->blocked_tasks[0], &rnp->blocked_tasks[2]);
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- list_splice_init(&rnp->blocked_tasks[1], &rnp->blocked_tasks[3]);
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- must_wait = rcu_preempted_readers_exp(rnp);
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+ if (!list_empty(&rnp->blkd_tasks)) {
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+ rnp->exp_tasks = rnp->blkd_tasks.next;
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+ must_wait = 1;
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+ }
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raw_spin_unlock(&rnp->lock); /* irqs remain disabled */
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if (!must_wait)
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rcu_report_exp_rnp(rsp, rnp);
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@@ -661,9 +694,7 @@ sync_rcu_preempt_exp_init(struct rcu_state *rsp, struct rcu_node *rnp)
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/*
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* Wait for an rcu-preempt grace period, but expedite it. The basic idea
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* is to invoke synchronize_sched_expedited() to push all the tasks to
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- * the ->blocked_tasks[] lists, move all entries from the first set of
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- * ->blocked_tasks[] lists to the second set, and finally wait for this
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- * second set to drain.
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+ * the ->blkd_tasks lists and wait for this list to drain.
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*/
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void synchronize_rcu_expedited(void)
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{
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@@ -695,7 +726,7 @@ void synchronize_rcu_expedited(void)
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if ((ACCESS_ONCE(sync_rcu_preempt_exp_count) - snap) > 0)
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goto unlock_mb_ret; /* Others did our work for us. */
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- /* force all RCU readers onto blocked_tasks[]. */
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+ /* force all RCU readers onto ->blkd_tasks lists. */
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synchronize_sched_expedited();
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raw_spin_lock_irqsave(&rsp->onofflock, flags);
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@@ -707,7 +738,7 @@ void synchronize_rcu_expedited(void)
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raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
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}
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- /* Snapshot current state of ->blocked_tasks[] lists. */
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+ /* Snapshot current state of ->blkd_tasks lists. */
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rcu_for_each_leaf_node(rsp, rnp)
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sync_rcu_preempt_exp_init(rsp, rnp);
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if (NUM_RCU_NODES > 1)
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@@ -715,7 +746,7 @@ void synchronize_rcu_expedited(void)
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raw_spin_unlock_irqrestore(&rsp->onofflock, flags);
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- /* Wait for snapshotted ->blocked_tasks[] lists to drain. */
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+ /* Wait for snapshotted ->blkd_tasks lists to drain. */
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rnp = rcu_get_root(rsp);
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wait_event(sync_rcu_preempt_exp_wq,
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sync_rcu_preempt_exp_done(rnp));
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Paul E. McKenney