[tip:sched/urgent] init, sched: Fix race between init and kthreadd

From: tip-bot for Peter Zijlstra
Date: Mon Jun 28 2010 - 16:26:33 EST

Commit-ID: b433c3d4549ae74935b585115f076c6fb7bc48fe
Gitweb: http://git.kernel.org/tip/b433c3d4549ae74935b585115f076c6fb7bc48fe
Author: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate: Mon, 28 Jun 2010 16:51:01 +0200
Committer: Ingo Molnar <mingo@xxxxxxx>
CommitDate: Mon, 28 Jun 2010 18:21:30 +0200

init, sched: Fix race between init and kthreadd

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>
Acked-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
LKML-Reference: <1277736661.3561.110.camel@laptop>
Signed-off-by: Ingo Molnar <mingo@xxxxxxx>
init/main.c | 12 ++++++++++++
1 files changed, 12 insertions(+), 0 deletions(-)

diff --git a/init/main.c b/init/main.c
index 3bdb152..633442f 100644
--- a/init/main.c
+++ b/init/main.c
@@ -422,18 +422,26 @@ 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);

@@ -855,6 +863,10 @@ 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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