mirror of
https://github.com/torvalds/linux
synced 2024-09-06 18:01:05 +00:00
![]() 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> |
||
---|---|---|
arch | ||
block | ||
certs | ||
crypto | ||
Documentation | ||
drivers | ||
fs | ||
include | ||
init | ||
ipc | ||
kernel | ||
lib | ||
LICENSES | ||
mm | ||
net | ||
samples | ||
scripts | ||
security | ||
sound | ||
tools | ||
usr | ||
virt | ||
.clang-format | ||
.cocciconfig | ||
.get_maintainer.ignore | ||
.gitattributes | ||
.gitignore | ||
.mailmap | ||
COPYING | ||
CREDITS | ||
Kbuild | ||
Kconfig | ||
MAINTAINERS | ||
Makefile | ||
README |
Linux kernel ============ There are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. Please read Documentation/admin-guide/README.rst first. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. The formatted documentation can also be read online at: https://www.kernel.org/doc/html/latest/ There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.