On Thu, May 09, 2019 at 02:09:03PM +0200, Daniel Vetter wrote:
> Fix this by creating a prinkt_safe_up() which calls wake_up_process
> outside of the spinlock. This isn't correct in full generality, but
> good enough for console_lock:
> 
> - console_lock doesn't use interruptible or killable or timeout down()
>   calls, hence an up() is the only thing that can wake up a process.

Wrong :/ Any task can be woken at any random time. We must, at all
times, assume spurious wakeups will happen.

> +void printk_safe_up(struct semaphore *sem)
> +{
> +     unsigned long flags;
> +     struct semaphore_waiter *waiter = NULL;
> +
> +     raw_spin_lock_irqsave(&sem->lock, flags);
> +     if (likely(list_empty(&sem->wait_list))) {
> +             sem->count++;
> +     } else {
> +             waiter = list_first_entry(&sem->wait_list,
> +                                       struct semaphore_waiter, list);
> +             list_del(&waiter->list);
> +             waiter->up = true;
> +     }
> +     raw_spin_unlock_irqrestore(&sem->lock, flags);
> +
> +     if (waiter) /* protected by being sole wake source */
> +             wake_up_process(waiter->task);
> +}
> +EXPORT_SYMBOL(printk_safe_up);

Since its only used from printk, that EXPORT really isn't needed.

Something like the below might work.

---
 kernel/locking/semaphore.c | 26 +++++++++++++-------------
 1 file changed, 13 insertions(+), 13 deletions(-)

diff --git a/kernel/locking/semaphore.c b/kernel/locking/semaphore.c
index 561acdd39960..ac0a67e95aac 100644
--- a/kernel/locking/semaphore.c
+++ b/kernel/locking/semaphore.c
@@ -38,7 +38,6 @@ static noinline void __down(struct semaphore *sem);
 static noinline int __down_interruptible(struct semaphore *sem);
 static noinline int __down_killable(struct semaphore *sem);
 static noinline int __down_timeout(struct semaphore *sem, long timeout);
-static noinline void __up(struct semaphore *sem);
 
 /**
  * down - acquire the semaphore
@@ -178,14 +177,24 @@ EXPORT_SYMBOL(down_timeout);
  */
 void up(struct semaphore *sem)
 {
+       struct semaphore_waiter *waiter;
+       DEFINE_WAKE_Q(wake_q);
        unsigned long flags;
 
        raw_spin_lock_irqsave(&sem->lock, flags);
-       if (likely(list_empty(&sem->wait_list)))
+       if (likely(list_empty(&sem->wait_list))) {
                sem->count++;
-       else
-               __up(sem);
+               goto unlock;
+       }
+
+       waiter = list_first_entry(&sem->wait_list, struct semaphore_waiter, 
list);
+       list_del(&waiter->list);
+       waiter->up = true;
+       wake_q_add(&wake_q, waiter->task);
+unlock:
        raw_spin_unlock_irqrestore(&sem->lock, flags);
+
+       wake_up_q(&wake_q);
 }
 EXPORT_SYMBOL(up);
 
@@ -252,12 +261,3 @@ static noinline int __sched __down_timeout(struct 
semaphore *sem, long timeout)
 {
        return __down_common(sem, TASK_UNINTERRUPTIBLE, timeout);
 }
-
-static noinline void __sched __up(struct semaphore *sem)
-{
-       struct semaphore_waiter *waiter = list_first_entry(&sem->wait_list,
-                                               struct semaphore_waiter, list);
-       list_del(&waiter->list);
-       waiter->up = true;
-       wake_up_process(waiter->task);
-}
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