[PATCH] init: Fix race between init and kthreadd -v3

Peter Zijlstra
2010-06-28T14:51:36Z

On Mon, 2010-06-28 at 16:19 +0200, Ingo Molnar wrote:

> I think you may be using a mutex as a completion in essence. Why not use
> completions instead?

Totally forgot about those things.. Yes they fit perfectly.

---
Subject: init: Fix race between init and kthreadd -v2

Ilya reported that on a very slow machine he could reliably reproduce a
race between forking init and kthreadd. We first fork init so that it
obtains pid-1, however since the scheduler is already fully running at
this point it can preempt and run the init thread before we spawn and
set kthreadd_task.

The init thread can then attempt spawning kthreads without kthreadd
being present which results in an OOPS.

Reported-by: Ilya Loginov <isloginov@gmail.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
---
init/main.c | 12 ++++++++++++
1 files changed, 12 insertions(+), 0 deletions(-)

diff --git a/init/main.c b/init/main.c
index e2a2bf3..2280f63 100644
--- a/init/main.c
+++ b/init/main.c
@@ -420,18 +420,26 @@ static void __init setup_command_line(char *command_line)
* gcc-3.4 accidentally inlines this function, so use noinline.
*/

+static __initdata DECLARE_COMPLETION(kthreadd_done);
+
static noinline void __init_refok rest_init(void)
__releases(kernel_lock)
{
int pid;

rcu_scheduler_starting();
+ /*
+ * We need to spawn init first so that it obtains pid-1, however
+ * the init task will end up wanting to create kthreads, which, if
+ * we schedule it before we create kthreadd, will OOPS.
+ */
kernel_thread(kernel_init, NULL, CLONE_FS | CLONE_SIGHAND);
numa_default_policy();
pid = kernel_thread(kthreadd, NULL, CLONE_FS | CLONE_FILES);
rcu_read_lock();
kthreadd_task = find_task_by_pid_ns(pid, &init_pid_ns);
rcu_read_unlock();
+ complete(&kthreadd_done);
unlock_kernel();

/*
@@ -847,6 +855,10 @@ static noinline int init_post(void)

static int __init kernel_init(void * unused)
{
+ /*
+ * Wait until kthreadd is all set-up.
+ */
+ wait_for_completion(&kthreadd_done);
lock_kernel();

/*