On 27 June 2013 03:27, Rafael J. Wysocki <rjw@xxxxxxx> wrote:
> Well, now, seeing that the locking around this seems to be kind of haphazard,
> I'm wondering what prevents two different threads from doing CPUFREQ_PRECHANGE
> concurrently in such a way that thread A will check transition_ongoing
> and thread B will check transition_ongoing and then both will set it if it
> was 'false' before. And then one of them will trigger the WARN() in
> CPUFREQ_POSTCHANGE.
>
> Is there any protection in place and if so then how does it work?
cpufreq_notify_transition() is called from driver->target() which is
called from __cpufreq_driver_target(). __cpufreq_driver_target()
is called directly by governors and cpufreq_driver_target() otherwise.
cpufreq_driver_target() implements proper locking and so it is fine.
__cpufreq_driver_target() is called from governors. From governors
it is is serialized in the sense two threads wouldn't call it at the same
time.
And so I thought this will work. But I just found a mistake in my code.
For multi-socket platforms with clock domains for sockets/clusters,
a single instance of transition_ongoing isn't enough and so this must
be embedded in struct cpufreq_policy.
Below patch must get this fixed (Attached).
-------------x---------------------x-----------------
From: Viresh Kumar <viresh.kumar@xxxxxxxxxx>
Date: Wed, 19 Jun 2013 10:16:55 +0530
Subject: [PATCH] cpufreq: make sure frequency transitions are serialized
Whenever we are changing frequency of a cpu, we are calling PRECHANGE and
POSTCHANGE notifiers. They must be serialized. i.e. PRECHANGE or POSTCHANGE
shouldn't be called twice contiguously.
This can happen due to bugs in users of __cpufreq_driver_target() or actual
cpufreq drivers who are sending these notifiers.
This patch adds some protection against this. Now, we keep track of the last
transaction and see if something went wrong.
Signed-off-by: Viresh Kumar <viresh.kumar@xxxxxxxxxx>
---
drivers/cpufreq/cpufreq.c | 14 ++++++++++++++
include/linux/cpufreq.h | 1 +
2 files changed, 15 insertions(+)
diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
index 2d53f47..75715f1 100644
--- a/drivers/cpufreq/cpufreq.c
+++ b/drivers/cpufreq/cpufreq.c
@@ -264,6 +264,12 @@ void __cpufreq_notify_transition(struct
cpufreq_policy *policy,
switch (state) {
case CPUFREQ_PRECHANGE:
+ if (WARN(policy->transition_ongoing,
+ "In middle of another frequency transition\n"))
+ return;
+
+ policy->transition_ongoing = true;
+
/* detect if the driver reported a value as "old frequency"
* which is not equal to what the cpufreq core thinks is
* "old frequency".
@@ -283,6 +289,12 @@ void __cpufreq_notify_transition(struct
cpufreq_policy *policy,
break;
case CPUFREQ_POSTCHANGE:
+ if (WARN(!policy->transition_ongoing,
+ "No frequency transition in progress\n"))
+ return;
+
+ policy->transition_ongoing = false;
+
adjust_jiffies(CPUFREQ_POSTCHANGE, freqs);
pr_debug("FREQ: %lu - CPU: %lu", (unsigned long)freqs->new,
(unsigned long)freqs->cpu);
@@ -1458,6 +1470,8 @@ int __cpufreq_driver_target(struct cpufreq_policy *policy,
if (cpufreq_disabled())
return -ENODEV;
+ if (policy->transition_ongoing)
+ return -EBUSY;
/* Make sure that target_freq is within supported range */
if (target_freq > policy->max)
diff --git a/include/linux/cpufreq.h b/include/linux/cpufreq.h
index 037d36a..8c13a45 100644
--- a/include/linux/cpufreq.h
+++ b/include/linux/cpufreq.h
@@ -115,6 +115,7 @@ struct cpufreq_policy {
struct kobject kobj;
struct completion kobj_unregister;
+ bool transition_ongoing; /* Tracks transition status */
};
#define CPUFREQ_ADJUST (0)
Attachment:
0001-cpufreq-make-sure-frequency-transitions-are-serializ.patch
Description: Binary data
