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Date: Mon, 18 Sep 2017 10:39:59 -0700 From: Bo Yan <byan@...dia.com> To: Viresh Kumar <viresh.kumar@...aro.org> CC: <rjw@...ysocki.net>, <linux-pm@...r.kernel.org>, <linux-kernel@...r.kernel.org> Subject: Re: [PATCH] cpufreq: cpufreq_stats: make last_index signed int On 09/17/2017 06:50 PM, Viresh Kumar wrote: > On 15-09-17, 13:13, Bo Yan wrote: >> It is possible for last_index to get a -1 if current frequency >> is not found in the freq table when stats is created. If the >> function "cpufreq_stats_update" is called before last_index is >> updated with a valid value, the "-1" will be used as index to >> update stats->time_in_state, triggering an exception. > > No, that's not how it works AFAIK and if it did, then can you explain how your > solution fixes it? > > AFAIK, what happens right now is that stats->last_index eventually stores > 2147483647 (uint max) and the exception comes while accessing that value. > > While with your change, it will become -1 and accessing array[-1] is fine by C > standards, though it is still the wrong thing to do as you are accessing > something outside of the array. > > We should just check last_index == -1 before calling cpufreq_stats_update(), > which is already done by one of the callers. > Currently, the "last_index" is being checked before cpufreq_stats_update(stats) inside function "cpufreq_stats_record_transition", so it's taken care of. However, the function "show_time_in_state" also calls cpufreq_stats_update, the similar check should be done there too, like this: diff --git a/drivers/cpufreq/cpufreq_stats.c b/drivers/cpufreq/cpufreq_stats.c index e75880eb037d..15305b5ec322 100644 --- a/drivers/cpufreq/cpufreq_stats.c +++ b/drivers/cpufreq/cpufreq_stats.c @@ -62,7 +62,8 @@ static ssize_t show_time_in_state(struct cpufreq_policy *policy, char *buf) if (policy->fast_switch_enabled) return 0; - cpufreq_stats_update(stats); + if ((int)stats->last_index >= 0) + cpufreq_stats_update(stats); for (i = 0; i < stats->state_num; i++) { len += sprintf(buf + len, "%u %llu\n", stats->freq_table[i], (unsigned long long) This is only needed when policy->cur is not in frequency table when stats table is created, in which case, stats->last_index will get -1, then user does a "cat time_in_state" before any frequency transition. Does this make sense?
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