linux-kernel - Re: [PATCH] cpufreq: Make cpufreq_online() call driver->offline() on errors

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Message-ID: <20210622062052.jo2by44djlyjpn5w@vireshk-i7>
Date:   Tue, 22 Jun 2021 11:50:52 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     Linux PM <linux-pm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] cpufreq: Make cpufreq_online() call driver->offline() on
 errors

On 21-06-21, 19:26, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> 
> In the CPU removal path the ->offline() callback provided by the
> driver is always invoked before ->exit(),

Note that exit() doesn't get called if offline() is present in the CPU
removal path. We call exit() _only_ when the cpufreq driver gets
unregistered.

> but in the cpufreq_online()
> error path it is not, so ->exit() is somehow expected to know the
> context in which it has been called and act accordingly.

I agree, it isn't very clear.

> That is less than straightforward, so make cpufreq_online() invoke
> the driver's ->offline() callback before ->exit() too.
> 
> This only potentially affects intel_pstate at this point.
> 
> Fixes: 91a12e91dc39 ("cpufreq: Allow light-weight tear down and bring up of CPUs")
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> ---
>  drivers/cpufreq/cpufreq.c |    3 +++
>  1 file changed, 3 insertions(+)
> 
> Index: linux-pm/drivers/cpufreq/cpufreq.c
> ===================================================================
> --- linux-pm.orig/drivers/cpufreq/cpufreq.c
> +++ linux-pm/drivers/cpufreq/cpufreq.c
> @@ -1516,6 +1516,9 @@ out_destroy_policy:
>  	up_write(&policy->rwsem);
>  
>  out_exit_policy:
> +	if (cpufreq_driver->offline)
> +		cpufreq_driver->offline(policy);
> +
>  	if (cpufreq_driver->exit)
>  		cpufreq_driver->exit(policy);

If we want to go down this path, then we better do more and make it
very explicit that ->offline() follows a previous invocation of
->online().

Otherwise, with above we will end up calling ->offline() for two separate
states, ->online() failed (i.e. two calls to offline() one after the other
here) and other generic failures after ->init() passed.

So, better make it clear that online/offline are paired like
init/exit.

diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
index 1d1b563cea4b..0ce48dcb2e8a 100644
--- a/drivers/cpufreq/cpufreq.c
+++ b/drivers/cpufreq/cpufreq.c
@@ -1347,14 +1347,11 @@ static int cpufreq_online(unsigned int cpu)
        }
 
        if (!new_policy && cpufreq_driver->online) {
-               ret = cpufreq_driver->online(policy);
-               if (ret) {
-                       pr_debug("%s: %d: initialization failed\n", __func__,
-                                __LINE__);
-                       goto out_exit_policy;
-               }
-
-               /* Recover policy->cpus using related_cpus */
+               /*
+                * We did light-weight tear down earlier, don't need to do heavy
+                * initialization here. Just recover policy->cpus using
+                * related_cpus.
+                */
                cpumask_copy(policy->cpus, policy->related_cpus);
        } else {
                cpumask_copy(policy->cpus, cpumask_of(cpu));
@@ -1378,6 +1375,13 @@ static int cpufreq_online(unsigned int cpu)
                cpumask_copy(policy->related_cpus, policy->cpus);
        }
 
+       ret = cpufreq_driver->online(policy);
+       if (ret) {
+               pr_debug("%s: %d: %d: ->online() failed\n", __func__, __LINE__,
+                        ret);
+               goto out_exit_policy;
+       }
+
        down_write(&policy->rwsem);
        /*
         * affected cpus must always be the one, which are online. We aren't
@@ -1518,6 +1522,9 @@ static int cpufreq_online(unsigned int cpu)
 
        up_write(&policy->rwsem);
 
+       if (cpufreq_driver->offline)
+               cpufreq_driver->offline(policy);
+
 out_exit_policy:
        if (cpufreq_driver->exit)
                cpufreq_driver->exit(policy);


-- 
viresh