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

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Message-ID: <20130813164315.GB32719@htj.dyndns.org>
Date:	Tue, 13 Aug 2013 12:43:15 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Stephen Boyd <sboyd@...eaurora.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()

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?

> 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()?

Thanks.

-- 
tejun
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