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

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Date:	Tue, 10 Mar 2015 09:37:08 -0700
From:	Jason Low <jason.low2@...com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Davidlohr Bueso <dave@...olabs.net>,
	LKML <linux-kernel@...r.kernel.org>, jason.low2@...com
Subject: Re: [PATCH] locking/mutex: Refactor mutex_spin_on_owner()

On Tue, 2015-03-10 at 09:11 +0100, Ingo Molnar wrote:
> * 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'.

Okay, I can update this patch. Should we make another similar update for
the rwsem then?

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