path: root/kernel/rcu/update.c
diff options
authorPaul E. McKenney <>2014-08-04 06:10:23 -0700
committerPaul E. McKenney <>2014-09-07 16:27:22 -0700
commit3f95aa81d265223fdb13ea2b59883766a05adbdf (patch)
tree5b6d2c42aaf8b20397bd09c0ac31738618f57046 /kernel/rcu/update.c
parent53c6d4edf874d3cbc031a53738c6cba9277faea5 (diff)
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 <> Suggested-by: Lai Jiangshan <> Signed-off-by: Paul E. McKenney <>
Diffstat (limited to 'kernel/rcu/update.c')
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. */
+/* 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)
+ * 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.