kthread: kill task_get_live_kthread()

task_get_live_kthread() looks confusing and unneeded.  It does
get_task_struct() but only kthread_stop() needs this, it can be called
even if the calller doesn't have a reference when we know that this
kthread can't exit until we do kthread_stop().

kthread_park() and kthread_unpark() do not need get_task_struct(), the
callers already have the reference.  And it can not help if we can race
with the exiting kthread anyway, kthread_park() can hang forever in this
case.

Change kthread_park() and kthread_unpark() to use to_live_kthread(),
change kthread_stop() to do get_task_struct() by hand and remove
task_get_live_kthread().

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/kernel/kthread.c b/kernel/kthread.c
index a84dee2..9b12d65 100644 (file)
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -324,12 +324,6 @@ struct task_struct *kthread_create_on_cpu(int (*threadfn)(void *data),
        return p;
 }
 
-static struct kthread *task_get_live_kthread(struct task_struct *k)
-{
-       get_task_struct(k);
-       return to_live_kthread(k);
-}
-
 static void __kthread_unpark(struct task_struct *k, struct kthread *kthread)
 {
        clear_bit(KTHREAD_SHOULD_PARK, &kthread->flags);
@@ -356,11 +350,10 @@ static void __kthread_unpark(struct task_struct *k, struct kthread *kthread)
  */
 void kthread_unpark(struct task_struct *k)
 {
-       struct kthread *kthread = task_get_live_kthread(k);
+       struct kthread *kthread = to_live_kthread(k);
 
        if (kthread)
                __kthread_unpark(k, kthread);
-       put_task_struct(k);
 }
 
 /**
@@ -377,7 +370,7 @@ void kthread_unpark(struct task_struct *k)
  */
 int kthread_park(struct task_struct *k)
 {
-       struct kthread *kthread = task_get_live_kthread(k);
+       struct kthread *kthread = to_live_kthread(k);
        int ret = -ENOSYS;
 
        if (kthread) {
@@ -390,7 +383,6 @@ int kthread_park(struct task_struct *k)
                }
                ret = 0;
        }
-       put_task_struct(k);
        return ret;
 }
 
@@ -411,10 +403,13 @@ int kthread_park(struct task_struct *k)
  */
 int kthread_stop(struct task_struct *k)
 {
-       struct kthread *kthread = task_get_live_kthread(k);
+       struct kthread *kthread;
        int ret;
 
        trace_sched_kthread_stop(k);
+
+       get_task_struct(k);
+       kthread = to_live_kthread(k);
        if (kthread) {
                set_bit(KTHREAD_SHOULD_STOP, &kthread->flags);
                __kthread_unpark(k, kthread);
@@ -422,10 +417,9 @@ int kthread_stop(struct task_struct *k)
                wait_for_completion(&kthread->exited);
        }
        ret = k->exit_code;
-
        put_task_struct(k);
-       trace_sched_kthread_stop_ret(ret);
 
+       trace_sched_kthread_stop_ret(ret);
        return ret;
 }
 EXPORT_SYMBOL(kthread_stop);