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Subject[PATCH 0/6] workqueue: Make flush_workqueue() also watch flush_work()
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From: Lai Jiangshan <laijs@linux.alibaba.com>

Li reported a problem[1] that the sanity check in destroy_workqueue() catches
a WARNING due to drain_workqueue() doesn't wait for flush_work() to finish.

The warning logs are listed below:

WARNING: CPU: 0 PID: 19336 at kernel/workqueue.c:4430 destroy_workqueue+0x11a/0x2f0
*****
destroy_workqueue: test_workqueue9 has the following busy pwq
pwq 4: cpus=2 node=0 flags=0x0 nice=0 active=0/1 refcnt=2
in-flight: 5658:wq_barrier_func
Showing busy workqueues and worker pools:
*****

It shows that even after drain_workqueue() returns, the barrier work item
is still in flight and the pwq (and a worker) is still busy on it.

The problem is caused by drain_workqueue() not watching flush_work():

Thread A Worker
/* normal work item with linked */
process_scheduled_works()
destroy_workqueue() process_one_work()
drain_workqueue() /* run normal work item */
/-- pwq_dec_nr_in_flight()
flush_workqueue() <---/
/* the last normal work item is done */
sanity_check process_one_work()
/-- raw_spin_unlock_irq(&pool->lock)
raw_spin_lock_irq(&pool->lock) <-/ /* maybe preempt */
*WARNING* wq_barrier_func()
/* maybe preempt by cond_resched() */

So the solution is to make drain_workqueue() watch for flush_work() which
means making flush_workqueue() watch for flush_work().

Due to historical convenience, we used WORK_NO_COLOR for barrier work items
queued by flush_work(). The color has two purposes:
Not participate in flushing
Not participate in nr_active

Only the second purpose is obligatory. So the plan is to mark barrier
work items inactive without using WORK_NO_COLOR in patch4 so that we can
assign a flushing color to them in patch5.

Patch1-3 are preparation, and patch6 is a cleanup.

[1]: https://lore.kernel.org/lkml/20210812083814.32453-1-lizhe.67@bytedance.com/

Lai Jiangshan (6):
workqueue: Rename "delayed" (delayed by active management) to
"inactive"
workqueue: Change arguement of pwq_dec_nr_in_flight()
workqueue: Change the code of calculating work_flags in
insert_wq_barrier()
workqueue: Mark barrier work with WORK_STRUCT_INACTIVE
workqueue: Assign a color to barrier work items
workqueue: Remove unused WORK_NO_COLOR

include/linux/workqueue.h | 13 ++--
kernel/workqueue.c | 133 ++++++++++++++++++++++--------------
kernel/workqueue_internal.h | 3 +-
3 files changed, 88 insertions(+), 61 deletions(-)

--
2.19.1.6.gb485710b

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