rcu: Make TASKS_RCU handle tasks that are almost done exiting

Once a task has passed exit_notify() in the do_exit() code path, it
is no longer on the task lists, and is therefore no longer visible
to rcu_tasks_kthread().  This means that an almost-exited task might
be preempted while within a trampoline, and this task won't be waited
on by rcu_tasks_kthread().  This commit fixes this bug by adding an
srcu_struct.  An exiting task does srcu_read_lock() just before calling
exit_notify(), and does the corresponding srcu_read_unlock() after
doing the final preempt_disable().  This means that rcu_tasks_kthread()
can do synchronize_srcu() to wait for all mostly-exited tasks to reach
their final preempt_disable() region, and then use synchronize_sched()
to wait for those tasks to finish exiting.

Reported-by: Oleg Nesterov <oleg@redhat.com>
Suggested-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 21 insertions, 0 deletions

diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index 5fd1ddbfcc55..403fc4ae539e 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -367,6 +367,13 @@ static struct rcu_head *rcu_tasks_cbs_head;
 static struct rcu_head **rcu_tasks_cbs_tail = &rcu_tasks_cbs_head;
 static DEFINE_RAW_SPINLOCK(rcu_tasks_cbs_lock);
 
+/* Track exiting tasks in order to allow them to be waited for. */
+DEFINE_SRCU(tasks_rcu_exit_srcu);
+
+/* Control stall timeouts.  Disable with <= 0, otherwise jiffies till stall. */
+static int rcu_task_stall_timeout __read_mostly = HZ * 60 * 3;
+module_param(rcu_task_stall_timeout, int, 0644);
+
 /* Post an RCU-tasks callback. */
 void call_rcu_tasks(struct rcu_head *rhp, void (*func)(struct rcu_head *rhp))
 {
@@ -518,6 +525,15 @@ static int __noreturn rcu_tasks_kthread(void *arg)
 		rcu_read_unlock();
 
 		/*
+		 * Wait for tasks that are in the process of exiting.
+		 * This does only part of the job, ensuring that all
+		 * tasks that were previously exiting reach the point
+		 * where they have disabled preemption, allowing the
+		 * later synchronize_sched() to finish the job.
+		 */
+		synchronize_srcu(&tasks_rcu_exit_srcu);
+
+		/*
 		 * Each pass through the following loop scans the list
 		 * of holdout tasks, removing any that are no longer
 		 * holdouts.  When the list is empty, we are done.
@@ -546,6 +562,11 @@ static int __noreturn rcu_tasks_kthread(void *arg)
 		 * ->rcu_tasks_holdout accesses to be within the grace
 		 * period, avoiding the need for memory barriers for
 		 * ->rcu_tasks_holdout accesses.
+		 *
+		 * In addition, this synchronize_sched() waits for exiting
+		 * tasks to complete their final preempt_disable() region
+		 * of execution, cleaning up after the synchronize_srcu()
+		 * above.
 		 */
 		synchronize_sched();