linux-kernel - Re: [PATCH] PM / QoS: Fix workqueue deadlock when using pm_qos_update_request

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Date:	Tue, 13 Aug 2013 09:46:26 -0700
From:	Stephen Boyd <sboyd@...eaurora.org>
To:	Tejun Heo <tj@...nel.org>
CC:	"Rafael J. Wysocki" <rjw@...k.pl>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] PM / QoS: Fix workqueue deadlock when using pm_qos_update_request_timeout()

On 08/13/13 09:43, Tejun Heo wrote:
> Hello, Stephen.
>
> On Thu, Aug 08, 2013 at 01:13:57PM -0700, Stephen Boyd wrote:
>> pm_qos_update_request_timeout() updates a qos and then schedules
>> a delayed work item to bring the qos back down to the default
>> after the timeout. When the work item runs, pm_qos_work_fn() will
>> call pm_qos_update_request() and deadlock because it tries to
>> cancel itself via cancel_delayed_work_sync(). Future callers of
>> that qos will also hang waiting to cancel the work that is
>> canceling itself. Before ed1ac6e (PM: don't use
>> [delayed_]work_pending(), 2013-01-11) this didn't happen because
>> the work function wouldn't try to cancel itself.
> I see.  That must have been racy tho.  If the work item execution
> races someone else queuing the work item, the same deadlock could
> happen, right?

Yes you're right. It was always racy.

>
>> Let's just do the little bit of pm_qos_update_request() here so
>> that we don't deadlock.
>>
>> Cc: Tejun Heo <tj@...nel.org>
>> Signed-off-by: Stephen Boyd <sboyd@...eaurora.org>
>> ---
>>  kernel/power/qos.c | 6 +++++-
>>  1 file changed, 5 insertions(+), 1 deletion(-)
>>
>> diff --git a/kernel/power/qos.c b/kernel/power/qos.c
>> index 06fe285..d52d314 100644
>> --- a/kernel/power/qos.c
>> +++ b/kernel/power/qos.c
>> @@ -308,7 +308,11 @@ static void pm_qos_work_fn(struct work_struct *work)
>>  						  struct pm_qos_request,
>>  						  work);
>>  
>> -	pm_qos_update_request(req, PM_QOS_DEFAULT_VALUE);
>> +	if (PM_QOS_DEFAULT_VALUE != req->node.prio)
>> +		pm_qos_update_target(
>> +				pm_qos_array[req->pm_qos_class]->constraints,
>> +				&req->node, PM_QOS_UPDATE_REQ,
>> +				PM_QOS_DEFAULT_VALUE);
> Maybe it'd be cleaner to add a param or internal variant of
> pm_qos_update_request()?

Maybe, but I was trying to make a minimal fix here.

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