linux-kernel - Re: [PATCH] cpuidle:powernv: Make the snooze timeout dynamic.

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Date:   Fri, 1 Jun 2018 10:24:33 +0530
From:   Gautham R Shenoy <ego@...ux.vnet.ibm.com>
To:     Balbir Singh <bsingharora@...il.com>
Cc:     "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Stewart Smith <stewart@...ux.vnet.ibm.com>,
        Michael Neuling <mikey@...ling.org>,
        Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
        Shilpasri G Bhat <shilpa.bhat@...ux.vnet.ibm.com>,
        Akshay Adiga <akshay.adiga@...ux.vnet.ibm.com>,
        Nicholas Piggin <npiggin@...il.com>,
        "open list:LINUX FOR POWERPC (32-BIT AND 64-BIT)" 
        <linuxppc-dev@...ts.ozlabs.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-pm@...r.kernel.org
Subject: Re: [PATCH] cpuidle:powernv: Make the snooze timeout dynamic.

Hi Balbir,

Thanks for reviewing the patch!

On Fri, Jun 01, 2018 at 12:51:05AM +1000, Balbir Singh wrote:
> On Thu, May 31, 2018 at 10:15 PM, Gautham R. Shenoy

[..snip..]
> >
> > +static u64 get_snooze_timeout(struct cpuidle_device *dev,
> > +                             struct cpuidle_driver *drv,
> > +                             int index)
> > +{
> > +       int i;
> > +
> > +       if (unlikely(!snooze_timeout_en))
> > +               return default_snooze_timeout;
> > +
> > +       for (i = index + 1; i < drv->state_count; i++) {
> > +               struct cpuidle_state *s = &drv->states[i];
> > +               struct cpuidle_state_usage *su = &dev->states_usage[i];
> > +
> > +               if (s->disabled || su->disable)
> > +                       continue;
> > +
> > +               return s->target_residency * tb_ticks_per_usec;
> 
> Can we ensure this is not prone to overflow?

s->target_residency is an "unsigned int" so can take a maximum value
of UINT_MAX. tb_ticks_per_usec is an "unsigned long" with a value in
the range of 100-1000. The return value is a u64. The product of
s->target_residency and tb_ticks_per_usec should be contained in u64.

Is there a potential case of overflow that I am missing ?

> 
> Otherwise looks good
> 
> Reviewed-by: Balbir Singh <bsingharora@...il.com>

--
Thanks and Regards
gautham.