Re: [PATCH v3] lock/semaphore: Avoid an unnecessary deadlock within up()

[Ingo Molnar]

* Byungchul Park <byungchul.park@xxxxxxx> wrote:

> diff --git a/kernel/locking/semaphore.c b/kernel/locking/semaphore.c
> index b8120ab..6634b68 100644
> --- a/kernel/locking/semaphore.c
> +++ b/kernel/locking/semaphore.c
> @@ -130,13 +130,14 @@ EXPORT_SYMBOL(down_killable);
>  int down_trylock(struct semaphore *sem)
>  {
>  	unsigned long flags;
> -	int count;
> +	int count = -1;
>  
> -	raw_spin_lock_irqsave(&sem->lock, flags);
> -	count = sem->count - 1;
> -	if (likely(count >= 0))
> -		sem->count = count;
> -	raw_spin_unlock_irqrestore(&sem->lock, flags);
> +	if (raw_spin_trylock_irqsave(&sem->lock, flags)) {
> +		count = sem->count - 1;
> +		if (likely(count >= 0))
> +			sem->count = count;
> +		raw_spin_unlock_irqrestore(&sem->lock, flags);
> +	}

I still don't really like it: two parallel trylocks will cause one of them to fail 
- while with the previous code they would both succeed.

None of these changes are necessary with all the printk robustification 
changes/enhancements we talked about, right?

Thanks,

	Ingo