On 06/28/2013 01:14 PM, Sergey Senozhatsky wrote:
> On (06/28/13 10:13), Viresh Kumar wrote:
>> On 26 June 2013 02:45, Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx> wrote:
>>>
>>> [ 60.277396] ======================================================
>>> [ 60.277400] [ INFO: possible circular locking dependency detected ]
>>> [ 60.277407] 3.10.0-rc7-dbg-01385-g241fd04-dirty #1744 Not tainted
>>> [ 60.277411] -------------------------------------------------------
>>> [ 60.277417] bash/2225 is trying to acquire lock:
>>> [ 60.277422] ((&(&j_cdbs->work)->work)){+.+...}, at: [<ffffffff810621b5>] flush_work+0x5/0x280
>>> [ 60.277444]
>>> but task is already holding lock:
>>> [ 60.277449] (cpu_hotplug.lock){+.+.+.}, at: [<ffffffff81042d8b>] cpu_hotplug_begin+0x2b/0x60
>>> [ 60.277465]
>>> which lock already depends on the new lock.
>>
>> Hi Sergey,
>>
>> Can you try reverting this patch?
>>
>> commit 2f7021a815f20f3481c10884fe9735ce2a56db35
>> Author: Michael Wang <wangyun@xxxxxxxxxxxxxxxxxx>
>> Date: Wed Jun 5 08:49:37 2013 +0000
>>
>> cpufreq: protect 'policy->cpus' from offlining during __gov_queue_work()
>>
>
> Hello,
> Yes, this helps, of course, but at the same time it returns the previous
> problem -- preventing cpu_hotplug in some places.
>
>
> I have a bit different (perhaps naive) RFC patch and would like to hear
> comments.
>
>
>
> The idead is to brake existing lock dependency chain by not holding
> cpu_hotplug lock mutex across the calls. In order to detect active
> refcount readers or active writer, refcount now may have the following
> values:
>
> -1: active writer -- only one writer may be active, readers are blocked
> 0: no readers/writer
>> 0: active readers -- many readers may be active, writer is blocked
>
> "blocked" reader or writer goes to wait_queue. as soon as writer finishes
> (refcount becomes 0), it wakeups all existing processes in a wait_queue.
> reader perform wakeup call only when it sees that pending writer is present
> (active_writer is not NULL).
>
> cpu_hotplug lock now only required to protect refcount cmp, inc, dec
> operations so it can be changed to spinlock.
>
Its best to avoid changing the core infrastructure in order to fix some
call-site, unless that scenario is really impossible to handle with the
current infrastructure.
I have a couple of suggestions below, to solve this issue, without touching
the core hotplug code:
You can perhaps try cancelling the work item in two steps:
a. using cancel_delayed_work() under CPU_DOWN_PREPARE
b. using cancel_delayed_work_sync() under CPU_POST_DEAD
And of course, destroy the resources associated with that work (like
the timer_mutex) only after the full tear-down.
Or perhaps you might find a way to perform the tear-down in just one step
at the CPU_POST_DEAD stage. Whatever works correctly.
The key point here is that the core CPU hotplug code provides us with the
CPU_POST_DEAD stage, where the hotplug lock is _not_ held. Which is exactly
what you want in solving the issue with cpufreq.
Regards,
Srivatsa S. Bhat
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