[PATCH] kthread: to_live_kthread() needs try_get_task_stack()

[Oleg Nesterov]

On 06/29, Andy Lutomirski wrote:
>
> I pushed that change to my tree (seems to work well enough to boot
> without warnings as long as I don't unmount XFS, but not particularly
> well tested).  Want to refresh your patch on top?

Please see the trivial fix below. Compile tested, but looks obvious.

Btw, why free_thread_stack() calls vfree() with irqs disabled? Doesn't
look good and perhaps even wrong; at least vmap_debug_free_range() does
flush_tlb_kernel_range() and smp_call_function() can deadlock?

-------------------------------------------------------------------------------
Subject: [PATCH] kthread: to_live_kthread() needs try_get_task_stack()

get_task_struct(tsk) no longer pins tsk->stack so all users of
to_live_kthread() should do try_get_task_stack/put_task_stack to protect
"struct kthread" which lives on kthread's stack.

TODO: Kill to_live_kthread(), perhaps we can even kill "struct kthread" too,
and rework kthread_stop(), it can use task_work_add() to sync with the exiting
kernel thread.

Signed-off-by: Oleg Nesterov <o...@redhat.com>
---
 kernel/kthread.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/kernel/kthread.c b/kernel/kthread.c
index 9ff173d..4ab4c37 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -64,7 +64,7 @@ static inline struct kthread *to_kthread(struct task_struct 
*k)
 static struct kthread *to_live_kthread(struct task_struct *k)
 {
        struct completion *vfork = ACCESS_ONCE(k->vfork_done);
-       if (likely(vfork))
+       if (likely(vfork) && try_get_task_stack(k))
                return __to_kthread(vfork);
        return NULL;
 }
@@ -425,8 +425,10 @@ void kthread_unpark(struct task_struct *k)
 {
        struct kthread *kthread = to_live_kthread(k);
 
-       if (kthread)
+       if (kthread) {
                __kthread_unpark(k, kthread);
+               put_task_stack(k);
+       }
 }
 EXPORT_SYMBOL_GPL(kthread_unpark);
 
@@ -455,6 +457,7 @@ int kthread_park(struct task_struct *k)
                                wait_for_completion(&kthread->parked);
                        }
                }
+               put_task_stack(k);
                ret = 0;
        }
        return ret;
@@ -490,6 +493,7 @@ int kthread_stop(struct task_struct *k)
                __kthread_unpark(k, kthread);
                wake_up_process(k);
                wait_for_completion(&kthread->exited);
+               put_task_stack(k);
        }
        ret = k->exit_code;
        put_task_struct(k);
-- 
2.5.0