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

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Date:	Wed, 26 Mar 2014 17:13:12 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Viresh Kumar <viresh.kumar@...aro.org>
CC:	tglx@...utronix.de, 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 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.

>  	if (base != new_base) {
>  		/*
> 

Regards,
Srivatsa S. Bhat


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