kthread: Ensure struct kthread is present for all kthreads

Today the rules are a bit iffy and arbitrary about which kernel
threads have struct kthread present.  Both idle threads and thread
started with create_kthread want struct kthread present so that is
effectively all kernel threads.  Make the rule that if PF_KTHREAD
and the task is running then struct kthread is present.

This will allow the kernel thread code to using tsk->exit_code
with different semantics from ordinary processes.

To make ensure that struct kthread is present for all
kernel threads move it's allocation into copy_process.

Add a deallocation of struct kthread in exec for processes
that were kernel threads.

Move the allocation of struct kthread for the initial thread
earlier so that it is not repeated for each additional idle
thread.

Move the initialization of struct kthread into set_kthread_struct
so that the structure is always and reliably initailized.

Clear set_child_tid in free_kthread_struct to ensure the kthread
struct is reliably freed during exec.  The function
free_kthread_struct does not need to clear vfork_done during exec as
exec_mm_release called from exec_mmap has already cleared vfork_done.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

1 files changed, 8 insertions, 8 deletions

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 3c9b0fda64ac..0404a8c572a1 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -8599,14 +8599,6 @@ void __init init_idle(struct task_struct *idle, int cpu)
 
 	__sched_fork(0, idle);
 
-	/*
-	 * The idle task doesn't need the kthread struct to function, but it
-	 * is dressed up as a per-CPU kthread and thus needs to play the part
-	 * if we want to avoid special-casing it in code that deals with per-CPU
-	 * kthreads.
-	 */
-	set_kthread_struct(idle);
-
 	raw_spin_lock_irqsave(&idle->pi_lock, flags);
 	raw_spin_rq_lock(rq);
 
@@ -9428,6 +9420,14 @@ void __init sched_init(void)
 	enter_lazy_tlb(&init_mm, current);
 
 	/*
+	 * The idle task doesn't need the kthread struct to function, but it
+	 * is dressed up as a per-CPU kthread and thus needs to play the part
+	 * if we want to avoid special-casing it in code that deals with per-CPU
+	 * kthreads.
+	 */
+	WARN_ON(set_kthread_struct(current));
+
+	/*
 	 * Make us the idle thread. Technically, schedule() should not be
 	 * called from this thread, however somewhere below it might be,
 	 * but because we are the idle thread, we just pick up running again