linux-kernel - Re: [PATCH] locking/mutex: Refactor mutex_spin_on

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Date:	Tue, 10 Mar 2015 09:11:48 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Jason Low <jason.low2@...com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Davidlohr Bueso <dave@...olabs.net>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] locking/mutex: Refactor mutex_spin_on_owner()


* Jason Low <jason.low2@...com> wrote:

> This patch applies on top of tip.
> 
> -------------------------------------------------------------------
> Similar to what Linus suggested for rwsem_spin_on_owner(), in
> mutex_spin_on_owner(), instead of having while (true) and breaking
> out of the spin loop on lock->owner != owner, we can have the loop
> directly check for while (lock->owner == owner). This improves the
> readability of the code.
> 
> Signed-off-by: Jason Low <jason.low2@...com>
> ---
>  kernel/locking/mutex.c |   17 +++++------------
>  1 files changed, 5 insertions(+), 12 deletions(-)
> 
> diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
> index 16b2d3c..1c3b7c5 100644
> --- a/kernel/locking/mutex.c
> +++ b/kernel/locking/mutex.c
> @@ -224,16 +224,8 @@ ww_mutex_set_context_slowpath(struct ww_mutex *lock,
>  static noinline
>  bool mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner)
>  {
> -	bool ret;
> -
>  	rcu_read_lock();
> -	while (true) {
> -		/* Return success when the lock owner changed */
> -		if (lock->owner != owner) {
> -			ret = true;
> -			break;
> -		}
> -
> +	while (lock->owner == owner) {
>  		/*
>  		 * Ensure we emit the owner->on_cpu, dereference _after_
>  		 * checking lock->owner still matches owner, if that fails,
> @@ -242,16 +234,17 @@ bool mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner)
>  		 */
>  		barrier();
>  
> +		/* Stop spinning when need_resched or owner is not running. */
>  		if (!owner->on_cpu || need_resched()) {
> -			ret = false;
> -			break;
> +			rcu_read_unlock();
> +			return false;
>  		}
>  
>  		cpu_relax_lowlatency();
>  	}
>  	rcu_read_unlock();
>  
> -	return ret;
> +	return true;

A nit: having multiple return statements in a function is not the 
cleanest approach, especially when we are holding locks.

It's better to add an 'out_unlock' label to before the 
rcu_read_unlock() and use that plus 'ret'.

Thanks,

	Ingo
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