linux-kernel - Re: [PATCH v2 5/5] cpufreq: Catch double invocations of cpufreq_freq_transition

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Date:	Tue, 29 Apr 2014 10:25:43 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Meelis Roos <mroos@...ux.ee>,
	"cpufreq@...r.kernel.org" <cpufreq@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 5/5] cpufreq: Catch double invocations of cpufreq_freq_transition_begin/end

On 29 April 2014 10:21, Viresh Kumar <viresh.kumar@...aro.org> wrote:
> Nice effort.
>
> On 29 April 2014 00:25, Srivatsa S. Bhat
> <srivatsa.bhat@...ux.vnet.ibm.com> wrote:
>> Now all such drivers have been fixed, but debugging this issue was not
>> very straight-forward (even lockdep didn't catch this). So let us add a
>> debug infrastructure to the cpufreq core to catch such issues more easily
>> in the future.
>
> BUT, I am not sure if we really need it :(
>
> I think we just got into the 'barrier' stuff as we had some doubts about it
> earlier and were quite sure that nothing else could go wrong. Otherwise
> the only problem could have been present was the second queuing
> from the same thread. And we will surely get stuck if that happens and
> we can't just miss that error..
>
>> Scenario 1: (Deadlock-free)
>> ----------
>>
>>          Task A                                         Task B
>>
>>     /* 1st freq transition */
>>     Invoke _begin() {
>>             ...
>>             ...
>>     }
>>
>>     Change the frequency
>>
>>                                                 Got interrupt for successful
>>                                                 change of frequency.
>>
>>                                                 /* 1st freq transition */
>>                                                 Invoke _end() {
>>                                                         ...
>>                                                         ...
>>     /* 2nd freq transition */                           ...
>>     Invoke _begin() {                                   ...
>>             ... //waiting for B                         ...
>>             ... //to finish _end()              }
>>             ...
>>             ...
>>     }
>>
>>
>> This scenario is actually deadlock-free because Task A can wait inside the
>> second call to _begin() without self-deadlocking, because it is the
>> responsibility of Task B to finish the first sequence by invoking the
>> corresponding _end().

WTF, I was writing my mail and it just got send due to some stupid combination
of keys :( .. Sorry.

Also, this might not work as expected. Consider this scenario:

     /* 1st freq transition */
     Invoke _begin() {
             ...
             ...
     }

     Start Change of frequency and return
     back as there is no end from same thread.

     /* 2nd freq transition as there is nobody stopping us */
     Invoke _begin() {
             ... //waiting for B
             ... //to finish _end()
             ...
             ...
     }

                                                              Got
interrupt for successful
                                                              change
of frequency.

                                                              /* 1st
freq transition */
                                                              Invoke _end() {
                                                                     ...
                                                                     ...
                                                             }

And your patch will probably break this ?
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