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Message-ID: <CAJZ5v0huMCZcqEkDw2KKp9RMBAU=rpex0zSAw_kK3CqjMyaibw@mail.gmail.com>
Date: Thu, 25 Jun 2020 12:52:45 +0200
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Viresh Kumar <viresh.kumar@...aro.org>
Cc: Quentin Perret <qperret@...gle.com>,
"Rafael J. Wysocki" <rafael@...nel.org>,
"Rafael J. Wysocki" <rjw@...ysocki.net>,
Arnd Bergmann <arnd@...db.de>,
Michael Ellerman <mpe@...erman.id.au>,
Benjamin Herrenschmidt <benh@...nel.crashing.org>,
Paul Mackerras <paulus@...ba.org>,
Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Juri Lelli <juri.lelli@...hat.com>,
Vincent Guittot <vincent.guittot@...aro.org>,
linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Linux PM <linux-pm@...r.kernel.org>,
"Cc: Android Kernel" <kernel-team@...roid.com>,
Todd Kjos <tkjos@...gle.com>, adharmap@...eaurora.org
Subject: Re: [PATCH v2 2/2] cpufreq: Specify default governor on command line
On Thu, Jun 25, 2020 at 10:50 AM Viresh Kumar <viresh.kumar@...aro.org> wrote:
>
> On 24-06-20, 16:32, Quentin Perret wrote:
> > Right, but I must admit that, looking at this more, I'm getting a bit
> > confused with the overall locking for governors :/
> >
> > When in cpufreq_init_policy() we find a governor using
> > find_governor(policy->last_governor), what guarantees this governor is
> > not concurrently unregistered? That is, what guarantees this governor
> > doesn't go away between that find_governor() call, and the subsequent
> > call to try_module_get() in cpufreq_set_policy() down the line?
> >
> > Can we somewhat assume that whatever governor is referred to by
> > policy->last_governor will have a non-null refcount? Or are the
> > cpufreq_online() and cpufreq_unregister_governor() path mutually
> > exclusive? Or is there something else?
>
> This should be sufficient to fix pending issues I believe. Based over your
> patches.
LGTM, but can you post it in a new thread to let Patchwork pick it up?
> -------------------------8<-------------------------
> From: Viresh Kumar <viresh.kumar@...aro.org>
> Date: Thu, 25 Jun 2020 13:15:23 +0530
> Subject: [PATCH] cpufreq: Fix locking issues with governors
>
> The locking around governors handling isn't adequate currently. The list
> of governors should never be traversed without locking in place. Also we
> must make sure the governor isn't removed while it is still referenced
> by code.
>
> Reported-by: Quentin Perret <qperret@...gle.com>
> Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
> ---
> drivers/cpufreq/cpufreq.c | 59 ++++++++++++++++++++++++---------------
> 1 file changed, 36 insertions(+), 23 deletions(-)
>
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index 4b1a5c0173cf..dad6b85f4c89 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -624,6 +624,24 @@ static struct cpufreq_governor *find_governor(const char *str_governor)
> return NULL;
> }
>
> +static struct cpufreq_governor *get_governor(const char *str_governor)
> +{
> + struct cpufreq_governor *t;
> +
> + mutex_lock(&cpufreq_governor_mutex);
> + t = find_governor(str_governor);
> + if (!t)
> + goto unlock;
> +
> + if (!try_module_get(t->owner))
> + t = NULL;
> +
> +unlock:
> + mutex_unlock(&cpufreq_governor_mutex);
> +
> + return t;
> +}
> +
> static unsigned int cpufreq_parse_policy(char *str_governor)
> {
> if (!strncasecmp(str_governor, "performance", CPUFREQ_NAME_LEN))
> @@ -643,28 +661,14 @@ static struct cpufreq_governor *cpufreq_parse_governor(char *str_governor)
> {
> struct cpufreq_governor *t;
>
> - mutex_lock(&cpufreq_governor_mutex);
> -
> - t = find_governor(str_governor);
> - if (!t) {
> - int ret;
> -
> - mutex_unlock(&cpufreq_governor_mutex);
> -
> - ret = request_module("cpufreq_%s", str_governor);
> - if (ret)
> - return NULL;
> -
> - mutex_lock(&cpufreq_governor_mutex);
> + t = get_governor(str_governor);
> + if (t)
> + return t;
>
> - t = find_governor(str_governor);
> - }
> - if (t && !try_module_get(t->owner))
> - t = NULL;
> -
> - mutex_unlock(&cpufreq_governor_mutex);
> + if (request_module("cpufreq_%s", str_governor))
> + return NULL;
>
> - return t;
> + return get_governor(str_governor);
> }
>
> /**
> @@ -818,12 +822,14 @@ static ssize_t show_scaling_available_governors(struct cpufreq_policy *policy,
> goto out;
> }
>
> + mutex_lock(&cpufreq_governor_mutex);
> for_each_governor(t) {
> if (i >= (ssize_t) ((PAGE_SIZE / sizeof(char))
> - (CPUFREQ_NAME_LEN + 2)))
> - goto out;
> + break;
> i += scnprintf(&buf[i], CPUFREQ_NAME_PLEN, "%s ", t->name);
> }
> + mutex_unlock(&cpufreq_governor_mutex);
> out:
> i += sprintf(&buf[i], "\n");
> return i;
> @@ -1060,11 +1066,14 @@ static int cpufreq_init_policy(struct cpufreq_policy *policy)
> {
> struct cpufreq_governor *gov = NULL;
> unsigned int pol = CPUFREQ_POLICY_UNKNOWN;
> + bool put_governor = false;
> + int ret;
>
> if (has_target()) {
> /* Update policy governor to the one used before hotplug. */
> - gov = find_governor(policy->last_governor);
> + gov = get_governor(policy->last_governor);
> if (gov) {
> + put_governor = true;
> pr_debug("Restoring governor %s for cpu %d\n",
> policy->governor->name, policy->cpu);
> } else if (default_governor) {
> @@ -1091,7 +1100,11 @@ static int cpufreq_init_policy(struct cpufreq_policy *policy)
> return -ENODATA;
> }
>
> - return cpufreq_set_policy(policy, gov, pol);
> + ret = cpufreq_set_policy(policy, gov, pol);
> + if (put_governor)
> + module_put(gov->owner);
> +
> + return ret;
> }
>
> static int cpufreq_add_policy_cpu(struct cpufreq_policy *policy, unsigned int cpu)
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