kthread: Generalize pf_io_worker so it can point to struct kthread

The point of using set_child_tid to hold the kthread pointer was that
it already did what is necessary.  There are now restrictions on when
set_child_tid can be initialized and when set_child_tid can be used in
schedule_tail.  Which indicates that continuing to use set_child_tid
to hold the kthread pointer is a bad idea.

Instead of continuing to use the set_child_tid field of task_struct
generalize the pf_io_worker field of task_struct and use it to hold
the kthread pointer.

Rename pf_io_worker (which is a void * pointer) to worker_private so
it can be used to store kthreads struct kthread pointer.  Update the
kthread code to store the kthread pointer in the worker_private field.
Remove the places where set_child_tid had to be dealt with carefully
because kthreads also used it.

Link: https://lkml.kernel.org/r/CAHk-=wgtFAA9SbVYg0gR1tqPMC17-NYcs0GQkaYg1bGhh1uJQQ@mail.gmail.com
Link: https://lkml.kernel.org/r/87a6grvqy8.fsf_-_@email.froward.int.ebiederm.org
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

3 files changed, 7 insertions, 17 deletions

diff --git a/kernel/fork.c b/kernel/fork.c
index 0816be1bb044..6f0293cb29c9 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -950,7 +950,7 @@ static struct task_struct *dup_task_struct(struct task_struct *orig, int node)
 	tsk->splice_pipe = NULL;
 	tsk->task_frag.page = NULL;
 	tsk->wake_q.next = NULL;
-	tsk->pf_io_worker = NULL;
+	tsk->worker_private = NULL;
 
 	account_kernel_stack(tsk, 1);
 
@@ -2032,12 +2032,6 @@ static __latent_entropy struct task_struct *copy_process(
 		siginitsetinv(&p->blocked, sigmask(SIGKILL)|sigmask(SIGSTOP));
 	}
 
-	/*
-	 * This _must_ happen before we call free_task(), i.e. before we jump
-	 * to any of the bad_fork_* labels. This is to avoid freeing
-	 * p->set_child_tid which is (ab)used as a kthread's data pointer for
-	 * kernel threads (PF_KTHREAD).
-	 */
 	p->set_child_tid = (clone_flags & CLONE_CHILD_SETTID) ? args->child_tid : NULL;
 	/*
 	 * Clear TID on mm_release()?
diff --git a/kernel/kthread.c b/kernel/kthread.c
index c14707d15341..261a3c3b9c6c 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -72,7 +72,7 @@ enum KTHREAD_BITS {
 static inline struct kthread *to_kthread(struct task_struct *k)
 {
 	WARN_ON(!(k->flags & PF_KTHREAD));
-	return (__force void *)k->set_child_tid;
+	return k->worker_private;
 }
 
 /*
@@ -80,7 +80,7 @@ static inline struct kthread *to_kthread(struct task_struct *k)
  *
  * Per construction; when:
  *
- *   (p->flags & PF_KTHREAD) && p->set_child_tid
+ *   (p->flags & PF_KTHREAD) && p->worker_private
  *
  * the task is both a kthread and struct kthread is persistent. However
  * PF_KTHREAD on it's own is not, kernel_thread() can exec() (See umh.c and
@@ -88,7 +88,7 @@ static inline struct kthread *to_kthread(struct task_struct *k)
  */
 static inline struct kthread *__to_kthread(struct task_struct *p)
 {
-	void *kthread = (__force void *)p->set_child_tid;
+	void *kthread = p->worker_private;
 	if (kthread && !(p->flags & PF_KTHREAD))
 		kthread = NULL;
 	return kthread;
@@ -109,11 +109,7 @@ bool set_kthread_struct(struct task_struct *p)
 	init_completion(&kthread->parked);
 	p->vfork_done = &kthread->exited;
 
-	/*
-	 * We abuse ->set_child_tid to avoid the new member and because it
-	 * can't be wrongly copied by copy_process().
-	 */
-	p->set_child_tid = (__force void __user *)kthread;
+	p->worker_private = kthread;
 	return true;
 }
 
@@ -128,7 +124,7 @@ void free_kthread_struct(struct task_struct *k)
 #ifdef CONFIG_BLK_CGROUP
 	WARN_ON_ONCE(kthread && kthread->blkcg_css);
 #endif
-	k->set_child_tid = (__force void __user *)NULL;
+	k->worker_private = NULL;
 	kfree(kthread);
 }
 
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index d8adbea77be1..ee222b89c692 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4908,7 +4908,7 @@ asmlinkage __visible void schedule_tail(struct task_struct *prev)
 	finish_task_switch(prev);
 	preempt_enable();
 
-	if (!(current->flags & PF_KTHREAD) && current->set_child_tid)
+	if (current->set_child_tid)
 		put_user(task_pid_vnr(current), current->set_child_tid);
 
 	calculate_sigpending();