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Subject[PATCH wq/for-3.9] workqueue: make work->data point to pool after try_to_grab_pending()
From: Lai Jiangshan <laijs@cn.fujitsu.com>

We plan to use work->data pointing to cwq as the synchronization
invariant when determining whether a given work item is on a locked
pool or not, which requires work->data pointing to cwq only while the
work item is queued on the associated pool.

With delayed_work updated not to overload work->data for target
workqueue recording, the only case where we still have off-queue
work->data pointing to cwq is try_to_grab_pending() which doesn't
update work->data after stealing a queued work item. There's no
reason for try_to_grab_pending() to not update work->data to point to
the pool instead of cwq, like the normal execution does.

This patch adds set_work_pool_and_keep_pending() which makes
work->data point to pool instead of cwq but keeps the pending bit
unlike set_work_pool_and_clear_pending() (surprise!).

After this patch, it's guaranteed that only queued work items point to
cwqs.

This patch doesn't introduce any visible behavior change.

tj: Renamed the new helper function to match
set_work_pool_and_clear_pending() and rewrote the description.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
---
kernel/workqueue.c | 10 ++++++++++
1 file changed, 10 insertions(+)

--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -556,6 +556,13 @@ static void set_work_cwq(struct work_str
WORK_STRUCT_PENDING | WORK_STRUCT_CWQ | extra_flags);
}

+static void set_work_pool_and_keep_pending(struct work_struct *work,
+ int pool_id)
+{
+ set_work_data(work, (unsigned long)pool_id << WORK_OFFQ_POOL_SHIFT,
+ WORK_STRUCT_PENDING);
+}
+
static void set_work_pool_and_clear_pending(struct work_struct *work,
int pool_id)
{
@@ -1115,6 +1122,9 @@ static int try_to_grab_pending(struct wo
cwq_dec_nr_in_flight(get_work_cwq(work),
get_work_color(work));

+ /* work->data points to cwq iff queued, point to pool */
+ set_work_pool_and_keep_pending(work, pool->id);
+
spin_unlock(&pool->lock);
return 1;
}

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