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

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Date:	Wed, 17 Feb 2016 10:28:29 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Byungchul Park <byungchul.park@....com>
Cc:	willy@...ux.intel.com, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, akinobu.mita@...il.com, jack@...e.cz,
	sergey.senozhatsky.work@...il.com, peter@...leysoftware.com
Subject: Re: [PATCH v3] lock/semaphore: Avoid an unnecessary deadlock within
 up()


* Byungchul Park <byungchul.park@....com> 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