linux-kernel - Re: [PATCH 04/14] hrtimer: use base->index instead of basenum in switch_hrtimer

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Message-ID: <alpine.DEB.2.02.1403261830400.21095@ionos.tec.linutronix.de>
Date:	Wed, 26 Mar 2014 18:31:18 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
cc:	Viresh Kumar <viresh.kumar@...aro.org>,
	linaro-kernel@...ts.linaro.org, linux-kernel@...r.kernel.org,
	fweisbec@...il.com, linaro-networking@...aro.org
Subject: Re: [PATCH 04/14] hrtimer: use base->index instead of basenum in
 switch_hrtimer_base()

On Wed, 26 Mar 2014, Srivatsa S. Bhat wrote:
> On 03/26/2014 04:51 PM, Viresh Kumar wrote:
> > In switch_hrtimer_base() we have created a local variable basenum which is set
> > to base->index. This variable is used at only one place. It makes code more
> > readable if we remove this variable use base->index directly.
> > 
> 
> No, this doesn't look right. Note that the code can re-execute
> the assignment to new_base, by jumping to the 'again' label.
> See below.
> 
> > --- a/kernel/hrtimer.c
> > +++ b/kernel/hrtimer.c
> > @@ -202,11 +202,10 @@ switch_hrtimer_base(struct hrtimer *timer, struct hrtimer_clock_base *base,
> >  	struct hrtimer_cpu_base *new_cpu_base;
> >  	int this_cpu = smp_processor_id();
> >  	int cpu = get_nohz_timer_target(pinned);
> > -	int basenum = base->index;
> > 
> >  again:
> >  	new_cpu_base = &per_cpu(hrtimer_bases, cpu);
> > -	new_base = &new_cpu_base->clock_base[basenum];
> > +	new_base = &new_cpu_base->clock_base[base->index];
> > 
> 
> Further down, timer->base can be altered (and set to NULL too).
> So if we jump back to 'again', we'll end up in trouble.
> So I think its important to cache the value in basenum and
> use it.

That's irrelevant. base is not changing.
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