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Subject[PATCH 24/31] workqueue: prepare flush_workqueue() for dynamic creation and destrucion of unbound pool_workqueues
Date
Unbound pwqs (pool_workqueues) will be dynamically created and
destroyed with the scheduled unbound workqueue w/ custom attributes
support. This patch synchronizes pwq linking and unlinking against
flush_workqueue() so that its operation isn't disturbed by pwqs coming
and going.

Linking and unlinking a pwq into wq->pwqs is now protected also by
wq->flush_mutex and a new pwq's work_color is initialized to
wq->work_color during linking. This ensures that pwqs changes don't
disturb flush_workqueue() in progress and the new pwq's work coloring
stays in sync with the rest of the workqueue.

flush_mutex during unlinking isn't strictly necessary but it's simpler
to do it anyway.

Signed-off-by: Tejun Heo <tj@kernel.org>
---
kernel/workqueue.c | 25 +++++++++++++++++++++++--
1 file changed, 23 insertions(+), 2 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index e092cd5..0f0da59 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -122,6 +122,9 @@ enum {
* W: workqueue_lock protected.
*
* R: workqueue_lock protected for writes. Sched-RCU protected for reads.
+ *
+ * FR: wq->flush_mutex and workqueue_lock protected for writes. Sched-RCU
+ * protected for reads.
*/

/* struct worker is defined in workqueue_internal.h */
@@ -185,7 +188,7 @@ struct pool_workqueue {
int nr_active; /* L: nr of active works */
int max_active; /* L: max active works */
struct list_head delayed_works; /* L: delayed works */
- struct list_head pwqs_node; /* R: node on wq->pwqs */
+ struct list_head pwqs_node; /* FR: node on wq->pwqs */
struct list_head mayday_node; /* W: node on wq->maydays */

/*
@@ -214,7 +217,7 @@ struct wq_flusher {
struct workqueue_struct {
unsigned int flags; /* W: WQ_* flags */
struct pool_workqueue __percpu *cpu_pwqs; /* I: per-cpu pwq's */
- struct list_head pwqs; /* R: all pwqs of this wq */
+ struct list_head pwqs; /* FR: all pwqs of this wq */
struct list_head list; /* W: list of all workqueues */

struct mutex flush_mutex; /* protects wq flushing */
@@ -3395,9 +3398,16 @@ static void pwq_unbound_release_workfn(struct work_struct *work)
if (WARN_ON_ONCE(!(wq->flags & WQ_UNBOUND)))
return;

+ /*
+ * Unlink @pwq. Synchronization against flush_mutex isn't strictly
+ * necessary on release but do it anyway. It's easier to verify
+ * and consistent with the linking path.
+ */
+ mutex_lock(&wq->flush_mutex);
spin_lock_irq(&workqueue_lock);
list_del_rcu(&pwq->pwqs_node);
spin_unlock_irq(&workqueue_lock);
+ mutex_unlock(&wq->flush_mutex);

put_unbound_pool(pool);
call_rcu_sched(&pwq->rcu, rcu_free_pwq);
@@ -3425,7 +3435,18 @@ static void init_and_link_pwq(struct pool_workqueue *pwq,
INIT_LIST_HEAD(&pwq->mayday_node);
INIT_WORK(&pwq->unbound_release_work, pwq_unbound_release_workfn);

+ /*
+ * Link @pwq and set the matching work_color. This is synchronized
+ * with flush_mutex to avoid confusing flush_workqueue().
+ */
+ mutex_lock(&wq->flush_mutex);
+ spin_lock_irq(&workqueue_lock);
+
+ pwq->work_color = wq->work_color;
list_add_tail_rcu(&pwq->pwqs_node, &wq->pwqs);
+
+ spin_unlock_irq(&workqueue_lock);
+ mutex_unlock(&wq->flush_mutex);
}

static int alloc_and_link_pwqs(struct workqueue_struct *wq)
--
1.8.1.2


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