rcu: Make call_rcu_tasks() tolerate first call with irqs disabled

Currently, if the very first call to call_rcu_tasks() has irqs disabled,
it will create the rcu_tasks_kthread with irqs disabled, which will
result in a splat in the memory allocator, which kthread_run() invokes
with the expectation that irqs are enabled.

This commit fixes this problem by deferring kthread creation if called
with irqs disabled.  The first call to call_rcu_tasks() that has irqs
enabled will create the kthread.

This bug was detected by rcutorture changes that were motivated by
Iftekhar Ahmed's mutation-testing efforts.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 5 insertions, 2 deletions

diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index 3e888cd5a594..f0d8322bc3ec 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -528,6 +528,7 @@ static int rcu_task_stall_timeout __read_mostly = HZ * 60 * 10;
 module_param(rcu_task_stall_timeout, int, 0644);
 
 static void rcu_spawn_tasks_kthread(void);
+static struct task_struct *rcu_tasks_kthread_ptr;
 
 /*
  * Post an RCU-tasks callback.  First call must be from process context
@@ -537,6 +538,7 @@ void call_rcu_tasks(struct rcu_head *rhp, rcu_callback_t func)
 {
         unsigned long flags;
         bool needwake;
+        bool havetask = READ_ONCE(rcu_tasks_kthread_ptr);
 
         rhp->next = NULL;
         rhp->func = func;
@@ -545,7 +547,9 @@ void call_rcu_tasks(struct rcu_head *rhp, rcu_callback_t func)
         *rcu_tasks_cbs_tail = rhp;
         rcu_tasks_cbs_tail = &rhp->next;
         raw_spin_unlock_irqrestore(&rcu_tasks_cbs_lock, flags);
-        if (needwake) {
+        /* We can't create the thread unless interrupts are enabled. */
+        if ((needwake && havetask) ||
+            (!havetask && !irqs_disabled_flags(flags))) {
                 rcu_spawn_tasks_kthread();
                 wake_up(&rcu_tasks_cbs_wq);
         }
@@ -790,7 +794,6 @@ static int __noreturn rcu_tasks_kthread(void *arg)
 static void rcu_spawn_tasks_kthread(void)
 {
         static DEFINE_MUTEX(rcu_tasks_kthread_mutex);
-        static struct task_struct *rcu_tasks_kthread_ptr;
         struct task_struct *t;
 
         if (READ_ONCE(rcu_tasks_kthread_ptr)) {