rcu: Prevent initialization race in rcutorture kthreads

When you do something like "t = kthread_run(...)", it is possible that
the kthread will start running before the assignment to "t" happens.
If the child kthread expects to find a pointer to its task_struct in "t",
it will then be fatally disappointed.  This commit therefore switches
such cases to kthread_create() followed by wake_up_process(), guaranteeing
that the assignment happens before the child kthread starts running.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 6 insertions, 4 deletions

diff --git a/kernel/rcutorture.c b/kernel/rcutorture.c
index 61be03ba598d..aaa7b9f3532a 100644
--- a/kernel/rcutorture.c
+++ b/kernel/rcutorture.c
@@ -2029,14 +2029,15 @@ rcu_torture_init(void)
         /* Start up the kthreads. */
 
         VERBOSE_PRINTK_STRING("Creating rcu_torture_writer task");
-        writer_task = kthread_run(rcu_torture_writer, NULL,
-                                  "rcu_torture_writer");
+        writer_task = kthread_create(rcu_torture_writer, NULL,
+                                     "rcu_torture_writer");
         if (IS_ERR(writer_task)) {
                 firsterr = PTR_ERR(writer_task);
                 VERBOSE_PRINTK_ERRSTRING("Failed to create writer");
                 writer_task = NULL;
                 goto unwind;
         }
+        wake_up_process(writer_task);
         fakewriter_tasks = kzalloc(nfakewriters * sizeof(fakewriter_tasks[0]),
                                    GFP_KERNEL);
         if (fakewriter_tasks == NULL) {
@@ -2151,14 +2152,15 @@ rcu_torture_init(void)
         }
         if (shutdown_secs > 0) {
                 shutdown_time = jiffies + shutdown_secs * HZ;
-                shutdown_task = kthread_run(rcu_torture_shutdown, NULL,
-                                            "rcu_torture_shutdown");
+                shutdown_task = kthread_create(rcu_torture_shutdown, NULL,
+                                               "rcu_torture_shutdown");
                 if (IS_ERR(shutdown_task)) {
                         firsterr = PTR_ERR(shutdown_task);
                         VERBOSE_PRINTK_ERRSTRING("Failed to create shutdown");
                         shutdown_task = NULL;
                         goto unwind;
                 }
+                wake_up_process(shutdown_task);
         }
         i = rcu_torture_onoff_init();
         if (i != 0) {