Date:	Mon, 24 Feb 2014 12:23:13 -0800
From:	Saravana Kannan <skannan@...eaurora.org>
To:	Viresh Kumar <viresh.kumar@...aro.org>
CC:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-arm-msm@...r.kernel.org,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] cpufreq: Set policy to non-NULL only after all hotplug
 online work is done

On 02/24/2014 02:55 AM, Viresh Kumar wrote:
> On 24 February 2014 14:09,  <skannan@...eaurora.org> wrote:
>>
>> Srivatsa S. Bhat wrote:
>>> On 02/24/2014 12:27 PM, Saravana Kannan wrote:
>>>> The existing code sets the per CPU policy to a non-NULL value before all
>>>> the steps performed during the hotplug online path is done.
>>>> Specifically,
>>>> this is done before the policy min/max, governors, etc are initialized
>>>> for
>>>> the policy.  This in turn means that calls to cpufreq_cpu_get() return a
>>>> non-NULL policy before the policy/CPU is ready to be used.
>>>>
>>>> To fix this, move the update of per CPU policy to a valid value after
>>>> all
>>>> the initialization steps for the policy are completed.
>>>>
>>>> Example kernel panic without this fix:
>>>> [  512.146185] Unable to handle kernel NULL pointer dereference at
>>>> virtual address 00000020
>>>> [  512.146195] pgd = c0003000
>>>> [  512.146213] [00000020] *pgd=80000000004003, *pmd=00000000
>>>> [  512.146228] Internal error: Oops: 206 [#1] PREEMPT SMP ARM
>>>> <snip>
>>>> [  512.146297] PC is at __cpufreq_governor+0x10/0x1ac
>>>> [  512.146312] LR is at cpufreq_update_policy+0x114/0x150
>>>> <snip>
>>>> [  512.149740] ---[ end trace f23a8defea6cd706 ]---
>>>> [  512.149761] Kernel panic - not syncing: Fatal exception
>>>> [  513.152016] CPU0: stopping
>>>> [  513.154710] CPU: 0 PID: 7136 Comm: mpdecision Tainted: G      D W
>>>> 3.10.0-gd727407-00074-g979ede8 #396
>>>> <snip>
>>>> [  513.317224] [<c0afe180>] (notifier_call_chain+0x40/0x68) from
>>>> [<c02a23ac>] (__blocking_notifier_call_chain+0x40/0x58)
>>>> [  513.327809] [<c02a23ac>] (__blocking_notifier_call_chain+0x40/0x58)
>>>> from [<c02a23d8>] (blocking_notifier_call_chain+0x14/0x1c)
>>>> [  513.339182] [<c02a23d8>] (blocking_notifier_call_chain+0x14/0x1c)
>>>> from [<c0803c68>] (cpufreq_set_policy+0xd4/0x2b8)
>>>> [  513.349594] [<c0803c68>] (cpufreq_set_policy+0xd4/0x2b8) from
>>>> [<c0803e7c>] (cpufreq_init_policy+0x30/0x98)
>>>> [  513.359231] [<c0803e7c>] (cpufreq_init_policy+0x30/0x98) from
>>>> [<c0805a18>] (__cpufreq_add_dev.isra.17+0x4dc/0x7a4)
>>>> [  513.369560] [<c0805a18>] (__cpufreq_add_dev.isra.17+0x4dc/0x7a4) from
>>>> [<c0805d38>] (cpufreq_cpu_callback+0x58/0x84)
>>>> [  513.379978] [<c0805d38>] (cpufreq_cpu_callback+0x58/0x84) from
>>>> [<c0afe180>] (notifier_call_chain+0x40/0x68)
>>>> [  513.389704] [<c0afe180>] (notifier_call_chain+0x40/0x68) from
>>>> [<c02812dc>] (__cpu_notify+0x28/0x44)
>>>> [  513.398728] [<c02812dc>] (__cpu_notify+0x28/0x44) from [<c0aeed90>]
>>>> (_cpu_up+0xf4/0x1dc)
>>>> [  513.406797] [<c0aeed90>] (_cpu_up+0xf4/0x1dc) from [<c0aeeed4>]
>>>> (cpu_up+0x5c/0x78)
>>>> [  513.414357] [<c0aeeed4>] (cpu_up+0x5c/0x78) from [<c0aec808>]
>>>> (store_online+0x44/0x74)
>>>> [  513.422253] [<c0aec808>] (store_online+0x44/0x74) from [<c03a40f4>]
>>>> (sysfs_write_file+0x108/0x14c)
>>>> [  513.431195] [<c03a40f4>] (sysfs_write_file+0x108/0x14c) from
>>>> [<c03517d4>] (vfs_write+0xd0/0x180)
>>>> [  513.439958] [<c03517d4>] (vfs_write+0xd0/0x180) from [<c0351ca8>]
>>>> (SyS_write+0x38/0x68)
>>>> [  513.447947] [<c0351ca8>] (SyS_write+0x38/0x68) from [<c0205de0>]
>>>> (ret_fast_syscall+0x0/0x30)
>>>>
>>>> In this specific case, CPU0 set's CPU1's policy->governor in
>>>> cpufreq_init_policy() to NULL while CPU1 is using the policy->governor
>>>> in
>>>> __cpufreq_governor().
>>>>
>>>
>>> Whoa! That's horrible! Can you please explain how exactly you
>>> triggered this? I'm curious...
>>
>> Just call cpufreq_update_policy(cpu) on a CPU and have another thread
>> online/offline it.
>
> But would you do that? Means why would you call them this way?
> cpufreq_update_policy() shouldn't be called that way I believe..

I was simplifying the scenario that causes it. We change the min/max 
using ADJUST notifiers for multiple reasons -- thermal being one of them.

thermal/cpu_cooling is one example of it.

So, cpufreq_update_policy() can be called on any CPU. If that races with 
someone offlining a CPU and onlining it, you'll get this crash.

>>>> Signed-off-by: Saravana Kannan <skannan@...eaurora.org>
>>>> ---
>>>>   drivers/cpufreq/cpufreq.c | 10 +++++-----
>>>>   1 file changed, 5 insertions(+), 5 deletions(-)
>>>>
>>>> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
>>>> index cb003a6..d5ceb43 100644
>>>> --- a/drivers/cpufreq/cpufreq.c
>>>> +++ b/drivers/cpufreq/cpufreq.c
>>>> @@ -1109,11 +1109,6 @@ static int __cpufreq_add_dev(struct device *dev,
>>>> struct subsys_interface *sif,
>>>>               goto err_set_policy_cpu;
>>>>       }
>>>>
>>>> -    write_lock_irqsave(&cpufreq_driver_lock, flags);
>>>> -    for_each_cpu(j, policy->cpus)
>>>> -            per_cpu(cpufreq_cpu_data, j) = policy;
>>>> -    write_unlock_irqrestore(&cpufreq_driver_lock, flags);
>>>> -
>>>>       if (cpufreq_driver->get) {
>>>>               policy->cur = cpufreq_driver->get(policy->cpu);
>>>
>>> If you move the per-cpu init further down, then what happens to the
>>> cpufreq_generic_get() that gets invoked here by some of the drivers?
>>
>> While cpufreq_generic_get() was a good refactor, I think it's causing
>> unnecessary cyclic dependency. You need that function to not fail for a
>> policy to get added properly and you need a proper policy for the function
>> to work. I care more about fixing the panic than trying to keep
>> cpufreq_generic_get().
>
> Surely, I will like a solution which would keep this as is :)..
> cpufreq_generic_get() should pass..

The idea would exist, but we can just call cpufreq_generic_get() and 
pass it policy->clk if it is not NULL. Does that work for you?

>>> It will almost always fail (because policy will be NULL) and hence
>>> CPU online will be unsuccessful (though you wont observe it because
>>> the error code is not bubbled up to the CPU hotplug core; perhaps we
>>> should).
>>
>> Good catch. I actually hit this issue, fixed and test it on a 3.12 cpufreq
>> code base. Since this new function isn't there at that point, I missed it.
>> Even if I did use the latest kernel, I wouldn't have hit this issue,
>> because the MSM cpufreq driver doesn't use this function.
>>
>>>
>>> IMHO, we should fix the synchronization in cpufreq_init_policy().
>>> I notice cpufreq_update_policy() as well as __cpufreq_governor() in
>>> your stack trace, which means cpufreq_set_policy() was involved.
>>> cpufreq_update_policy() takes the policy->rwsem for write, whereas
>>> cpufreq_init_policy() doesn't take that lock at all, which is clearly
>>> wrong.
>>>
>>> My guess is that, if we fix that locking, everything will be fine and
>>> you won't hit the bug. Would you like to give that a shot?
>>
>> While locking might need fixing, this is not about the locking though.
>> Plenty of drivers and the framework use cpufreq_cpu_get() to get a policy
>> that they consider valid and also use it as a way to check and reject API
>> calls trying to manipulate an offline CPU.
>>
>> Without this fix, the framework is advertising an incomplete policy object
>> as available. I think that breaks the CPUfreq framework APIs in a very
>> fundamental sense. This is a "no-no" in programming.
>>
>> It's like trying to register a CPUfreq driver before getting the clocks to
>> control the CPU.
>
> So the real question here is: What fields of 'policy' do we need to guarantee
> to be initialized before policy is made available for others? And probably
> that is what we need to fix.

That's going to be hard to define since future uses could be looking at 
different fields. What is the API semantics of cpufreq_cpu_get(). I 
can't ever imagine it being correct for any API to return a partially 
initialized data structure.

> Even your current solution would break things. For example, below will happen
> before policy is set in per-cpu variable:
> - CPUFREQ_CREATE_POLICY notifier will be fired and calls to do cpufreq_cpu_get()
> there will fail. And hence cpufreq-stats sysfs will break.
I did this on 3.12. I'll fix it up to handle this one.

> - Governors also use cpufreq_cpu_get() and so those would also fail as they
> are started from cpufreq_init_policy()..
The only use of this in governors is in update_sampling_rate() and the 
behavior after this fix would be correct in that case. If the policy is 
not fully initialized -- that update doesn't get to go through.

All other uses of this API fall under one of these categories:
* CPUs are already fully offline whenever it's called.
* Already get the real policy as part of the notifier so don't need to
   call cpufreq_cpu_get

If I find any that doesn't fit the above, I would be glad to fix that if 
it's pointed out.

Regards,
Saravana
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