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authorLai Jiangshan <laijs@linux.alibaba.com>2021-08-17 09:32:38 +0800
committerTejun Heo <tj@kernel.org>2021-08-17 07:49:10 -1000
commitd812796eb3906424cd3c0eea530983961e2f88bd (patch)
treeffb809babfa728dd5cef8a0b9106c0d9a04d5d4b /kernel/workqueue.c
parent018f3a13dd6300701103f268b6bfec0a56beea57 (diff)
workqueue: Assign a color to barrier work items
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>
Diffstat (limited to 'kernel/workqueue.c')
-rw-r--r--kernel/workqueue.c20
1 files changed, 12 insertions, 8 deletions
diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 6657bcbd2064..33a6b4a2443d 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1197,10 +1197,6 @@ 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? */
@@ -1307,7 +1303,7 @@ 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.
@@ -2234,6 +2230,7 @@ __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
@@ -2339,6 +2336,7 @@ __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);
}
@@ -2682,7 +2680,8 @@ 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;
/*
@@ -2705,17 +2704,22 @@ 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);
}