sched,rcu: Rework try_invoke_on_locked_down_task()

Give try_invoke_on_locked_down_task() a saner name and have it return
an int so that the caller might distinguish between different reasons
of failure.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Paul E. McKenney <paulmck@kernel.org>
Acked-by: Vasily Gorbik <gor@linux.ibm.com>
Tested-by: Vasily Gorbik <gor@linux.ibm.com> # on s390
Link: https://lkml.kernel.org/r/20210929152428.649944917@infradead.org

include/linux/wait.h

@@ -1160,6 +1160,7 @@ int autoremove_wake_function(struct wait_queue_entry *wq_entry, unsigned mode, i
 		(wait)->flags = 0;						\
 	} while (0)
 
-bool try_invoke_on_locked_down_task(struct task_struct *p, bool (*func)(struct task_struct *t, void *arg), void *arg);
+typedef int (*task_call_f)(struct task_struct *p, void *arg);
+extern int task_call_func(struct task_struct *p, task_call_f func, void *arg);
 
 #endif /* _LINUX_WAIT_H */

kernel/rcu/tasks.h

@@ -928,7 +928,7 @@ reset_ipi:
 }
 
 /* Callback function for scheduler to check locked-down task.  */
-static bool trc_inspect_reader(struct task_struct *t, void *arg)
+static int trc_inspect_reader(struct task_struct *t, void *arg)
 {
 	int cpu = task_cpu(t);
 	bool in_qs = false;
@@ -939,7 +939,7 @@ static bool trc_inspect_reader(struct task_struct *t, void *arg)
 
 		// If no chance of heavyweight readers, do it the hard way.
 		if (!ofl && !IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB))
-			return false;
+			return -EINVAL;
 
 		// If heavyweight readers are enabled on the remote task,
 		// we can inspect its state despite its currently running.
@@ -947,7 +947,7 @@ static bool trc_inspect_reader(struct task_struct *t, void *arg)
 		n_heavy_reader_attempts++;
 		if (!ofl && // Check for "running" idle tasks on offline CPUs.
 		    !rcu_dynticks_zero_in_eqs(cpu, &t->trc_reader_nesting))
-			return false; // No quiescent state, do it the hard way.
+			return -EINVAL; // No quiescent state, do it the hard way.
 		n_heavy_reader_updates++;
 		if (ofl)
 			n_heavy_reader_ofl_updates++;
@@ -962,7 +962,7 @@ static bool trc_inspect_reader(struct task_struct *t, void *arg)
 	t->trc_reader_checked = true;
 
 	if (in_qs)
-		return true;  // Already in quiescent state, done!!!
+		return 0;  // Already in quiescent state, done!!!
 
 	// The task is in a read-side critical section, so set up its
 	// state so that it will awaken the grace-period kthread upon exit
@@ -970,7 +970,7 @@ static bool trc_inspect_reader(struct task_struct *t, void *arg)
 	atomic_inc(&trc_n_readers_need_end); // One more to wait on.
 	WARN_ON_ONCE(READ_ONCE(t->trc_reader_special.b.need_qs));
 	WRITE_ONCE(t->trc_reader_special.b.need_qs, true);
-	return true;
+	return 0;
 }
 
 /* Attempt to extract the state for the specified task. */
@@ -992,7 +992,7 @@ static void trc_wait_for_one_reader(struct task_struct *t,
 
 	// Attempt to nail down the task for inspection.
 	get_task_struct(t);
-	if (try_invoke_on_locked_down_task(t, trc_inspect_reader, NULL)) {
+	if (!task_call_func(t, trc_inspect_reader, NULL)) {
 		put_task_struct(t);
 		return;
 	}

kernel/rcu/tree_stall.h

@@ -240,16 +240,16 @@ struct rcu_stall_chk_rdr {
  * Report out the state of a not-running task that is stalling the
  * current RCU grace period.
  */
-static bool check_slow_task(struct task_struct *t, void *arg)
+static int check_slow_task(struct task_struct *t, void *arg)
 {
 	struct rcu_stall_chk_rdr *rscrp = arg;
 
 	if (task_curr(t))
-		return false; // It is running, so decline to inspect it.
+		return -EBUSY; // It is running, so decline to inspect it.
 	rscrp->nesting = t->rcu_read_lock_nesting;
 	rscrp->rs = t->rcu_read_unlock_special;
 	rscrp->on_blkd_list = !list_empty(&t->rcu_node_entry);
-	return true;
+	return 0;
 }
 
 /*
@@ -283,7 +283,7 @@ static int rcu_print_task_stall(struct rcu_node *rnp, unsigned long flags)
 	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 	while (i) {
 		t = ts[--i];
-		if (!try_invoke_on_locked_down_task(t, check_slow_task, &rscr))
+		if (task_call_func(t, check_slow_task, &rscr))
 			pr_cont(" P%d", t->pid);
 		else
 			pr_cont(" P%d/%d:%c%c%c%c",

kernel/sched/core.c

@@ -4110,7 +4110,7 @@ out:
 }
 
 /**
- * try_invoke_on_locked_down_task - Invoke a function on task in fixed state
+ * task_call_func - Invoke a function on task in fixed state
  * @p: Process for which the function is to be invoked, can be @current.
  * @func: Function to invoke.
  * @arg: Argument to function.
@@ -4123,12 +4123,12 @@ out:
  * Returns:
  *   Whatever @func returns
  */
-bool try_invoke_on_locked_down_task(struct task_struct *p, bool (*func)(struct task_struct *t, void *arg), void *arg)
+int task_call_func(struct task_struct *p, task_call_f func, void *arg)
 {
 	struct rq *rq = NULL;
 	unsigned int state;
 	struct rq_flags rf;
-	bool ret = false;
+	int ret;
 
 	raw_spin_lock_irqsave(&p->pi_lock, rf.flags);