Re: [PATCH] cpufreq: fix governor start/stop race condition

From: Xiaoguang Chen
Date: Wed May 22 2013 - 22:44:52 EST

On 05/22/2013 04:46 PM, Viresh Kumar wrote:
Sorry for being late buddy..

On 16 May 2013 11:44, Xiaoguang Chen <chenxg@xxxxxxxxxxx> wrote:
On 05/13/2013 06:47 PM, Xiaoguang Chen wrote:
Why is the mail came this way.. You forwarded it?
I didn't see your reponse, So I once replied this mail once.:)

cpufreq governor stop and start should be kept in sequence.
If not, there will be unexpected behavior, for example:

we have 4 cpus and policy->cpu=cpu0, cpu1/2/3 are linked to cpu0.
the normal sequence is as below:

1) Current governor is userspace, one application tries to set
governor to ondemand. it will call __cpufreq_set_policy in which it
will stop userspace governor and then start ondemand governor.

2) Current governor is userspace, now cpu0 hotplugs in cpu3, it will
call cpufreq_add_policy_cpu. on which it first stops userspace
governor, and then starts userspace governor.

Now if the sequence of above two cases interleaves, it becames
below sequence:

1) application stops userspace governor
2) hotplug stops userspace governor
3) application starts ondemand governor
4) hotplug starts a governor

in step 4, hotplug is supposed to start userspace governor, but now
the governor has been changed by application to ondemand, so hotplug
starts ondemand governor again !!!!

The solution is as below:
cpufreq policy has a rwsem to protect the read and write of policy.
make the scope of the rwsem to contain cpufreq governor stop/start
sequence, so that after the stop governor has started, other threads
will not stop governor, they have to wait the current thread starts
the governor and then do their job.

Change-Id: I054bb52789fc8abdcf80bdcc1caebd429c182bb0
Signed-off-by: Xiaoguang Chen <chenxg@xxxxxxxxxxx>
drivers/cpufreq/cpufreq.c | 19 ++++++++-----------
1 file changed, 8 insertions(+), 11 deletions(-)

diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
index 1b8a48e..935f750 100644
--- a/drivers/cpufreq/cpufreq.c
+++ b/drivers/cpufreq/cpufreq.c
@@ -811,14 +811,14 @@ static int cpufreq_add_policy_cpu(unsigned int cpu,
unsigned int sibling,
int ret = 0, has_target = !!cpufreq_driver->target;
unsigned long flags;
+ lock_policy_rwsem_write(sibling);
policy = cpufreq_cpu_get(sibling);
if (has_target)
__cpufreq_governor(policy, CPUFREQ_GOV_STOP);
We can't have locks are GOV_STOP earlier.. And now we can't have it
across *_EXIT.. Check latest code... As this gives some circular dependency
to locking and it fails.
Do you mean my patch will cause deadlock? I once tried to add another lock
to protect the GOV_STOP/START sequence instead of using the rwsem in this patch.
But I saw deadlock indeed.
In cpufreq_add_policy_cpu, the lock has to be added before the rwsem since GOV_STOP is
before lock_policy_rwsem_write, but in cpufreq_update_policy, it will first get the rwsem, and then
call __cpufreq_set_policy which will contain GOV_STOP again, if we add the new lock before this GOV_STOP,
then we may get deadlock in below sequence:
1) hotplug in one cpu by calling cpufreq_add_policy_cpu in which new lock is locked first then rwsem is locked.
2) governor change in cpufreq_update_policy in which rwsem is locked first then new lock is locked.
this is a deadlock issue if above two steps interleaves


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