linux-kernel - Re: [PATCH v4 1/2] cpufreq: fix race on cpufreq online

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Date:   Fri, 13 May 2022 11:43:43 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Schspa Shi <schspa@...il.com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 1/2] cpufreq: fix race on cpufreq online

On 13-05-22, 14:06, Schspa Shi wrote:
> Viresh Kumar <viresh.kumar@...aro.org> writes:
> > On 12-05-22, 21:52, Schspa Shi wrote:
> >> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> >> index 80f535cc8a75..35dffd738580 100644
> >> --- a/drivers/cpufreq/cpufreq.c
> >> +++ b/drivers/cpufreq/cpufreq.c
> >> @@ -953,7 +953,10 @@ static ssize_t show(struct kobject *kobj, struct attribute *attr, char *buf)
> >>              return -EIO;
> >>
> >>      down_read(&policy->rwsem);
> >> -    ret = fattr->show(policy, buf);
> >> +    if (unlikely(policy_is_inactive(policy)))
> >> +            ret = -EBUSY;
> >> +    else
> >> +            ret = fattr->show(policy, buf);
> >
> > I like it the way I have done earlier, initialize ret to -EBUSY and
> > get rid of the else part and call show/store in if itself. Same for
> > below.
> >
> 
> I add a unlikely here, to avoid branch prediction failed.

I am not asking you to drop it, I also added the unlikely within the
implementation of policy_is_inactive() then. It can be written as:

if (likely(!policy_is_inactive(policy)))
        ret = fattr->show(policy, buf);

> And move the
> to the fail path to avoid a register assignment to -EBUSY.

We don't care about such assignments for performance to be honest.
This makes the code smaller by few lines, that's enough.

-- 
viresh