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init, sched: Fix race between init and kthreadd

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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>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
LKML-Reference: <1277736661.3561.110.camel@laptop>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
This commit is contained in:
Peter Zijlstra 2010-06-28 16:51:01 +02:00 committed by Ingo Molnar
parent 0d98bb2656
commit b433c3d454
1 changed files with 12 additions and 0 deletions
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12
init/main.c
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@ -422,18 +422,26 @@ static void __init setup_command_line(char *command_line)
* gcc-3.4 accidentally inlines this function, so use noinline. * gcc-3.4 accidentally inlines this function, so use noinline.
*/ */
static __initdata DECLARE_COMPLETION(kthreadd_done);
static noinline void __init_refok rest_init(void) static noinline void __init_refok rest_init(void)
__releases(kernel_lock) __releases(kernel_lock)
{ {
int pid; int pid;
rcu_scheduler_starting(); 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); kernel_thread(kernel_init, NULL, CLONE_FS | CLONE_SIGHAND);
numa_default_policy(); numa_default_policy();
pid = kernel_thread(kthreadd, NULL, CLONE_FS | CLONE_FILES); pid = kernel_thread(kthreadd, NULL, CLONE_FS | CLONE_FILES);
rcu_read_lock(); rcu_read_lock();
kthreadd_task = find_task_by_pid_ns(pid, &init_pid_ns); kthreadd_task = find_task_by_pid_ns(pid, &init_pid_ns);
rcu_read_unlock(); rcu_read_unlock();
complete(&kthreadd_done);
unlock_kernel(); unlock_kernel();
/* /*
@ -855,6 +863,10 @@ static noinline int init_post(void)
static int __init kernel_init(void * unused) static int __init kernel_init(void * unused)
{ {
/*
* Wait until kthreadd is all set-up.
*/
wait_for_completion(&kthreadd_done);
lock_kernel(); lock_kernel();
/* /*