From: Lai Jiangshan laijs@linux.alibaba.com
mainline inclusion from mainline-v5.15-rc1 commit d812796eb3906424cd3c0eea530983961e2f88bd category: bugfix bugzilla: https://gitee.com/openeuler/kernel/issues/I7LRJF
Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?i...
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There was no strong reason to or not to flush barrier work items in flush_workqueue(). And we have to make barrier work items not participate in nr_active so we had been using WORK_NO_COLOR for them which also makes them can't be flushed by flush_workqueue().
And the users of flush_workqueue() often do not intend to wait barrier work items issued by flush_work(). That made the choice sound perfect.
But barrier work items have reference to internal structure (pool_workqueue) and the worker thread[s] is/are still busy for the workqueue user when the barrrier work items are not done. So it is reasonable to make flush_workqueue() also watch for flush_work() to make it more robust.
And a problem[1] reported by Li Zhe shows that we need such robustness. 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 flush_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() */
Thread A can get the pool lock after the Worker unlocks the pool lock before running wq_barrier_func(). And if there is any preemption happen around wq_barrier_func(), destroy_workqueue()'s sanity check is more likely to get the lock and catch it. (Note: preemption is not necessary to cause the bug, the unlocking is enough to possibly trigger the WARNING.)
A simple solution might be just executing all linked barrier work items once without releasing pool lock after the head work item's pwq_dec_nr_in_flight(). But this solution has two problems:
1) the head work item might also be barrier work item when the user-queued work item is cancelled. For example: thread 1: thread 2: queue_work(wq, &my_work) flush_work(&my_work) cancel_work_sync(&my_work); /* Neiter my_work nor the barrier work is scheduled. */ destroy_workqueue(wq); /* This is an easier way to catch the WARNING. */
2) there might be too much linked barrier work items and running them all once without releasing pool lock just causes trouble.
The only solution is to make flush_workqueue() aslo watch barrier work items. So we have to assign a color to these barrier work items which is the color of the head (user-queued) work item.
Assigning a color doesn't cause any problem in ative management, because the prvious patch made barrier work items not participate in nr_active via WORK_STRUCT_INACTIVE rather than reliance on the (old) WORK_NO_COLOR.
[1]: https://lore.kernel.org/lkml/20210812083814.32453-1-lizhe.67@bytedance.com/ Reported-by: Li Zhe lizhe.67@bytedance.com Signed-off-by: Lai Jiangshan laijs@linux.alibaba.com Signed-off-by: Tejun Heo tj@kernel.org Signed-off-by: Zeng Heng zengheng4@huawei.com --- kernel/workqueue.c | 20 ++++++++++++-------- kernel/workqueue_internal.h | 3 ++- 2 files changed, 14 insertions(+), 9 deletions(-)
diff --git a/kernel/workqueue.c b/kernel/workqueue.c index ef049b6a26a4..3ce68188ad0b 100644 --- a/kernel/workqueue.c +++ b/kernel/workqueue.c @@ -1166,10 +1166,6 @@ static void pwq_dec_nr_in_flight(struct pool_workqueue *pwq, unsigned long work_ } }
- /* uncolored work items don't participate in flushing */ - if (color == WORK_NO_COLOR) - goto out_put; - pwq->nr_in_flight[color]--;
/* is flush in progress and are we at the flushing tip? */ @@ -1272,7 +1268,7 @@ static int try_to_grab_pending(struct work_struct *work, bool is_dwork, * canceled (see the comments in insert_wq_barrier()). * * An inactive work item cannot be grabbed directly because - * it might have linked NO_COLOR work items which, if left + * it might have linked barrier work items which, if left * on the inactive_works list, will confuse pwq->nr_active * management later on and cause stall. Make sure the work * item is activated before grabbing. @@ -2191,6 +2187,7 @@ __acquires(&pool->lock) worker->current_func = work->func; worker->current_pwq = pwq; work_data = *work_data_bits(work); + worker->current_color = get_work_color(work_data);
/* * Record wq name for cmdline and debug reporting, may get @@ -2305,6 +2302,7 @@ __acquires(&pool->lock) worker->current_work = NULL; worker->current_func = NULL; worker->current_pwq = NULL; + worker->current_color = INT_MAX; pwq_dec_nr_in_flight(pwq, work_data); }
@@ -2648,7 +2646,8 @@ static void insert_wq_barrier(struct pool_workqueue *pwq, struct wq_barrier *barr, struct work_struct *target, struct worker *worker) { - unsigned int work_flags = work_color_to_flags(WORK_NO_COLOR); + unsigned int work_flags = 0; + unsigned int work_color; struct list_head *head;
/* @@ -2671,17 +2670,22 @@ static void insert_wq_barrier(struct pool_workqueue *pwq, * If @target is currently being executed, schedule the * barrier to the worker; otherwise, put it after @target. */ - if (worker) + if (worker) { head = worker->scheduled.next; - else { + work_color = worker->current_color; + } else { unsigned long *bits = work_data_bits(target);
head = target->entry.next; /* there can already be other linked works, inherit and set */ work_flags |= *bits & WORK_STRUCT_LINKED; + work_color = get_work_color(*bits); __set_bit(WORK_STRUCT_LINKED_BIT, bits); }
+ pwq->nr_in_flight[work_color]++; + work_flags |= work_color_to_flags(work_color); + debug_work_activate(&barr->work); insert_work(pwq, &barr->work, head, work_flags); } diff --git a/kernel/workqueue_internal.h b/kernel/workqueue_internal.h index 30cfed226b39..8d9f3c225414 100644 --- a/kernel/workqueue_internal.h +++ b/kernel/workqueue_internal.h @@ -30,7 +30,8 @@ struct worker {
struct work_struct *current_work; /* L: work being processed */ work_func_t current_func; /* L: current_work's fn */ - struct pool_workqueue *current_pwq; /* L: current_work's pwq */ + struct pool_workqueue *current_pwq; /* L: current_work's pwq */ + unsigned int current_color; /* L: current_work's color */ struct list_head scheduled; /* L: scheduled works */
/* 64 bytes boundary on 64bit, 32 on 32bit */