[PATCH] mm/slub: enable IRQs once scheduling is working

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

Pavel V. Panteleev reported that a WARN_ON_ONCE(p->migrate_disable <= 0)
triggered in 3.14.79-rt85 during boot from radix_tree_insert().
The problem is that radix_tree_insert() allocates memory and SLUB does
not enable interrupts in allocate_slab().
They can't be be enabled unconditionally because early in the boot
process they have to remain off. Enabling them at the "SYSTEM_RUNNING"
state is too late. The earliest point is once we have scheduling
working that is once the kthread is running.

In newer -RT (v4.11+) there is the check
	if (system_state > SYSTEM_BOOTING)

to enable interrupts in the allocator which is what this patch intends:
Once scheduling is working enable interrupts in the allocator path so we
can take sleeping locks.

Reported-by: "Pavel V. Panteleev" <panteleev_p@mcst.ru>
Tested-by: "Pavel V. Panteleev" <panteleev_p@mcst.ru>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 init/main.c | 2 ++
 mm/slub.c   | 3 ++-
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/init/main.c b/init/main.c
index 6470deef01c9..362761effeac 100644
--- a/init/main.c
+++ b/init/main.c
@@ -379,6 +379,7 @@ static void __init setup_command_line(char *command_line)
  */
 
 static __initdata DECLARE_COMPLETION(kthreadd_done);
+extern bool slab_do_irq_on;
 
 static noinline void __ref rest_init(void)
 {
@@ -396,6 +397,7 @@ static noinline void __ref rest_init(void)
 	rcu_read_lock();
 	kthreadd_task = find_task_by_pid_ns(pid, &init_pid_ns);
 	rcu_read_unlock();
+	slab_do_irq_on = true;
 	complete(&kthreadd_done);
 
 	/*
diff --git a/mm/slub.c b/mm/slub.c
index 67eb368b9314..22ec805130e5 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -1527,6 +1527,7 @@ static inline bool shuffle_freelist(struct kmem_cache *s, struct page *page)
 	return false;
 }
 #endif /* CONFIG_SLAB_FREELIST_RANDOM */
+bool slab_do_irq_on;
 
 static struct page *allocate_slab(struct kmem_cache *s, gfp_t flags, int node)
 {
@@ -1543,7 +1544,7 @@ static struct page *allocate_slab(struct kmem_cache *s, gfp_t flags, int node)
 	if (gfpflags_allow_blocking(flags))
 		enableirqs = true;
 #ifdef CONFIG_PREEMPT_RT_FULL
-	if (system_state == SYSTEM_RUNNING)
+	if (slab_do_irq_on)
 		enableirqs = true;
 #endif
 	if (enableirqs)
-- 
2.15.0