linux-kernel - Re: [PATCH 2/3] workqueues: Account unbound workqueue in a seperate list

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Date:	Sat, 15 Mar 2014 13:40:19 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Kevin Hilman <khilman@...aro.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Christoph Lameter <cl@...ux.com>,
	Mike Galbraith <bitbucket@...ine.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Tejun Heo <tj@...nel.org>,
	Viresh Kumar <viresh.kumar@...aro.org>
Subject: Re: [PATCH 2/3] workqueues: Account unbound workqueue in a seperate
 list

On Fri, Mar 14, 2014 at 11:17:35AM -0700, Kevin Hilman wrote:
> Frederic Weisbecker <fweisbec@...il.com> writes:
> 
> > The workqueues are all listed in a global list protected by a big mutex.
> > And this big mutex is used in apply_workqueue_attrs() as well.
> >
> > Now as we plan to implement a directory to control the cpumask of
> > all non-ABI unbound workqueues, we want to be able to iterate over all
> > unbound workqueues and call apply_workqueue_attrs() for each of
> > them with the new cpumask.
> >
> > But the risk for a deadlock is on the way: we need to iterate the list
> > of workqueues under wq_pool_mutex. But then apply_workqueue_attrs()
> > itself calls wq_pool_mutex.
> >
> > The easiest solution to work around this is to keep track of unbound
> > workqueues in a separate list with a separate mutex.
> >
> > It's not very pretty unfortunately.
> >
> > Cc: Christoph Lameter <cl@...ux.com>
> > Cc: Kevin Hilman <khilman@...aro.org>
> > Cc: Mike Galbraith <bitbucket@...ine.de>
> > Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > Cc: Tejun Heo <tj@...nel.org>
> > Cc: Viresh Kumar <viresh.kumar@...aro.org>
> > Not-Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> > ---
> >  kernel/workqueue.c | 15 +++++++++++++++
> >  1 file changed, 15 insertions(+)
> >
> > diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> > index 4d230e3..ad8f727 100644
> > --- a/kernel/workqueue.c
> > +++ b/kernel/workqueue.c
> > @@ -232,6 +232,7 @@ struct wq_device;
> >  struct workqueue_struct {
> >  	struct list_head	pwqs;		/* WR: all pwqs of this wq */
> >  	struct list_head	list;		/* PL: list of all workqueues */
> > +	struct list_head	unbound_list;	/* PL: list of unbound workqueues */
> >  
> >  	struct mutex		mutex;		/* protects this wq */
> >  	int			work_color;	/* WQ: current work color */
> > @@ -288,9 +289,11 @@ static bool wq_numa_enabled;		/* unbound NUMA affinity enabled */
> >  static struct workqueue_attrs *wq_update_unbound_numa_attrs_buf;
> >  
> >  static DEFINE_MUTEX(wq_pool_mutex);	/* protects pools and workqueues list */
> > +static DEFINE_MUTEX(wq_unbound_mutex);	/* protects list of unbound workqueues */
> >  static DEFINE_SPINLOCK(wq_mayday_lock);	/* protects wq->maydays list */
> >  
> >  static LIST_HEAD(workqueues);		/* PL: list of all workqueues */
> > +static LIST_HEAD(workqueues_unbound);	/* PL: list of unbound workqueues */
> >  static bool workqueue_freezing;		/* PL: have wqs started freezing? */
> >  
> >  /* the per-cpu worker pools */
> > @@ -4263,6 +4266,12 @@ struct workqueue_struct *__alloc_workqueue_key(const char *fmt,
> >  
> >  	mutex_unlock(&wq_pool_mutex);
> >  
> > +	if (wq->flags & WQ_UNBOUND) {
> > +		mutex_lock(&wq_unbound_mutex);
> > +		list_add(&wq->unbound_list, &workqueues_unbound);
> > +		mutex_unlock(&wq_unbound_mutex);
> > +	}
> > +
> >  	return wq;
> >  
> >  err_free_wq:
> > @@ -4318,6 +4327,12 @@ void destroy_workqueue(struct workqueue_struct *wq)
> >  	list_del_init(&wq->list);
> >  	mutex_unlock(&wq_pool_mutex);
> >  
> > +	if (wq->flags & WQ_UNBOUND) {
> > +		mutex_lock(&wq_unbound_mutex);
> > +		list_del(&wq->unbound_list);
> > +		mutex_unlock(&wq_unbound_mutex);
> > +	}
> > +
> >  	workqueue_sysfs_unregister(wq);
> >  
> >  	if (wq->rescuer) {
> 
> Looks good, except for minor nit: I think you're missing an init of the
> new list:
> 
> diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> index cc708f23d801..a01592f08321 100644
> --- a/kernel/workqueue.c
> +++ b/kernel/workqueue.c
> @@ -4309,6 +4309,7 @@ struct workqueue_struct
> *__alloc_workqueue_key(const char *fmt,
> 
>         lockdep_init_map(&wq->lockdep_map, lock_name, key, 0);
>         INIT_LIST_HEAD(&wq->list);
> +       INIT_LIST_HEAD(&wq->unbound_list);

Actually that's only for the head of a list. Nodes don't need such initialization.

Thanks.

> 
>         if (alloc_and_link_pwqs(wq) < 0)
>                 goto err_free_wq;
> 
> 
> Kevin
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