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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>
82 lines
2.4 KiB
C
82 lines
2.4 KiB
C
/* SPDX-License-Identifier: GPL-2.0 */
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/*
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* kernel/workqueue_internal.h
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*
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* Workqueue internal header file. Only to be included by workqueue and
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* core kernel subsystems.
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*/
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#ifndef _KERNEL_WORKQUEUE_INTERNAL_H
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#define _KERNEL_WORKQUEUE_INTERNAL_H
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#include <linux/workqueue.h>
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#include <linux/kthread.h>
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#include <linux/preempt.h>
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struct worker_pool;
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/*
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* The poor guys doing the actual heavy lifting. All on-duty workers are
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* either serving the manager role, on idle list or on busy hash. For
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* details on the locking annotation (L, I, X...), refer to workqueue.c.
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*
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* Only to be used in workqueue and async.
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*/
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struct worker {
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/* on idle list while idle, on busy hash table while busy */
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union {
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struct list_head entry; /* L: while idle */
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struct hlist_node hentry; /* L: while busy */
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};
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struct work_struct *current_work; /* L: work being processed */
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work_func_t current_func; /* L: current_work's fn */
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struct pool_workqueue *current_pwq; /* L: current_work's pwq */
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unsigned int current_color; /* L: current_work's color */
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struct list_head scheduled; /* L: scheduled works */
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/* 64 bytes boundary on 64bit, 32 on 32bit */
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struct task_struct *task; /* I: worker task */
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struct worker_pool *pool; /* A: the associated pool */
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/* L: for rescuers */
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struct list_head node; /* A: anchored at pool->workers */
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/* A: runs through worker->node */
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unsigned long last_active; /* L: last active timestamp */
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unsigned int flags; /* X: flags */
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int id; /* I: worker id */
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int sleeping; /* None */
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/*
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* Opaque string set with work_set_desc(). Printed out with task
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* dump for debugging - WARN, BUG, panic or sysrq.
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*/
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char desc[WORKER_DESC_LEN];
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/* used only by rescuers to point to the target workqueue */
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struct workqueue_struct *rescue_wq; /* I: the workqueue to rescue */
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/* used by the scheduler to determine a worker's last known identity */
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work_func_t last_func;
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};
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/**
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* current_wq_worker - return struct worker if %current is a workqueue worker
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*/
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static inline struct worker *current_wq_worker(void)
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{
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if (in_task() && (current->flags & PF_WQ_WORKER))
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return kthread_data(current);
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return NULL;
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}
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/*
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* Scheduler hooks for concurrency managed workqueue. Only to be used from
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* sched/ and workqueue.c.
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*/
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void wq_worker_running(struct task_struct *task);
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void wq_worker_sleeping(struct task_struct *task);
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work_func_t wq_worker_last_func(struct task_struct *task);
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#endif /* _KERNEL_WORKQUEUE_INTERNAL_H */
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