linux-kernel - Re: [PATCH] kernel/locking/semaphore: use wake

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Date:   Fri, 10 May 2019 14:50:53 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Daniel Vetter <daniel.vetter@...ll.ch>
Cc:     Intel Graphics Development <intel-gfx@...ts.freedesktop.org>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        Daniel Vetter <daniel.vetter@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will.deacon@....com>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        John Ogness <john.ogness@...utronix.de>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kernel/locking/semaphore: use wake_q in up()

On (05/09/19 22:06), Daniel Vetter wrote:
[..]
> +/* 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);
>  
>  	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);

So the new code still can printk/WARN under sem->lock in some buggy
cases.

E.g.
	wake_q_add()
	 get_task_struct()
	  refcount_inc_checked()
	   WARN_ONCE()

Are we fine with that?

	-ss