linux-kernel - [PATCH 3/3] cpufreq: Prevent problems in update_policy_cpu() if last_cpu == new

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Message-ID: <20130911201334.7832.49714.stgit@srivatsabhat.in.ibm.com>
Date:	Thu, 12 Sep 2013 01:43:42 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	rjw@...k.pl, swarren@...dotorg.org, viresh.kumar@...aro.org
Cc:	cpufreq@...r.kernel.org, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Subject: [PATCH 3/3] cpufreq: Prevent problems in update_policy_cpu() if
 last_cpu == new_cpu

If update_policy_cpu() is invoked with the existing policy->cpu itself
as the new-cpu parameter, then a lot of things can go terribly wrong.

In its present form, update_policy_cpu() always assumes that the new-cpu
is different from policy->cpu and invokes other functions to perform their
respective updates. And those functions implement the actual update like
this:

per_cpu(..., new_cpu) = per_cpu(..., last_cpu);
per_cpu(..., last_cpu) = NULL;

Thus, when new_cpu == last_cpu, the final NULL assignment makes the per-cpu
references vanish into thin air! (memory leak). From there, it leads to more
problems: cpufreq_stats_create_table() now doesn't find the per-cpu reference
and hence tries to create a new sysfs-group; but sysfs already had created
the group earlier, so it complains that it cannot create a duplicate filename.
In short, the repercussions of a rather innocuous invocation of
update_policy_cpu() can turn out to be pretty nasty.

Ideally update_policy_cpu() should handle this situation (new == last)
gracefully, and not lead to such severe problems. So fix it by adding an
appropriate check.

Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@...ux.vnet.ibm.com>
Tested-by: Stephen Warren <swarren@...dia.com>
---

 drivers/cpufreq/cpufreq.c |    3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
index 247842b..d32040c 100644
--- a/drivers/cpufreq/cpufreq.c
+++ b/drivers/cpufreq/cpufreq.c
@@ -949,6 +949,9 @@ static void cpufreq_policy_free(struct cpufreq_policy *policy)

 static void update_policy_cpu(struct cpufreq_policy *policy, unsigned int cpu)
 {
+	if (cpu == policy->cpu)
+		return;
+
 	policy->last_cpu = policy->cpu;
 	policy->cpu = cpu;

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