Re: [PATCH] kernel/locking/semaphore: use wake_q in up()

[Petr Mladek <pmladek@suse.com>]

On Thu 2019-05-09 22:06:33, Daniel Vetter wrote:
> console_trylock, called from within printk, can be called from pretty
> much anywhere. Including try_to_wake_up. Note that this isn't common,
> usually the box is in pretty bad shape at that point already. But it
> really doesn't help when then lockdep jumps in and spams the logs,
> potentially obscuring the real backtrace we're really interested in.
> One case I've seen (slightly simplified backtrace):
> 
> Fix this specific locking recursion by moving the wake_up_process out
> from under the semaphore.lock spinlock, using wake_q as recommended by
> Peter Zijlstra.

It might make sense to mention also the optimization effect mentioned
by Peter.

> diff --git a/kernel/locking/semaphore.c b/kernel/locking/semaphore.c
> index 561acdd39960..7a6f33715688 100644
> --- a/kernel/locking/semaphore.c
> +++ b/kernel/locking/semaphore.c
> @@ -169,6 +169,14 @@ int down_timeout(struct semaphore *sem, long timeout)
>  }
>  EXPORT_SYMBOL(down_timeout);
>  
> +/* Functions for the contended case */
> +
> +struct semaphore_waiter {
> +	struct list_head list;
> +	struct task_struct *task;
> +	bool up;
> +};
> +
>  /**
>   * up - release the semaphore
>   * @sem: the semaphore to release
> @@ -179,24 +187,25 @@ EXPORT_SYMBOL(down_timeout);
>  void up(struct semaphore *sem)
>  {
>  	unsigned long flags;
> +	struct semaphore_waiter *waiter;
> +	DEFINE_WAKE_Q(wake_q);

We need to call wake_q_init(&wake_q) to make sure that
it is empty.

Best Regards,
Petr

>  	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);
> +	} else {
> +		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);
> +	}
>  	raw_spin_unlock_irqrestore(&sem->lock, flags);
> +
> +	wake_up_q(&wake_q);
>  }
>  EXPORT_SYMBOL(up);
>  
> -/* Functions for the contended case */
> -
> -struct semaphore_waiter {
> -	struct list_head list;
> -	struct task_struct *task;
> -	bool up;
> -};
> -
>  /*
>   * Because this function is inlined, the 'state' parameter will be
>   * constant, and thus optimised away by the compiler.  Likewise the