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

From: Peter Zijlstra
Date: Mon Jun 28 2010 - 10:51:39 EST

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@xxxxxxxxx>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
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)
int pid;

+ /*
+ * 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);
pid = kernel_thread(kthreadd, NULL, CLONE_FS | CLONE_FILES);
kthreadd_task = find_task_by_pid_ns(pid, &init_pid_ns);
+ complete(&kthreadd_done);

@@ -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);


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