No description
Find a file
Lai Jiangshan d812796eb3 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>
2021-08-17 07:49:10 -10:00
arch ARM: 2021-07-29 09:42:09 -07:00
block block-5.14-2021-07-08 2021-07-09 12:05:33 -07:00
certs Kbuild updates for v5.13 (2nd) 2021-05-08 10:00:11 -07:00
crypto Merge branch 'linus' of git://git.kernel.org/pub/scm/linux/kernel/git/herbert/crypto-2.6 2021-07-09 11:00:44 -07:00
Documentation ARM: 2021-07-29 09:42:09 -07:00
drivers platform-drivers-x86 for v5.14-2 2021-07-28 10:31:17 -07:00
fs \n 2021-07-28 10:38:38 -07:00
include workqueue: Rename "delayed" (delayed by active management) to "inactive" 2021-08-17 07:49:09 -10:00
init Revert "mm/slub: use stackdepot to save stack trace in objects" 2021-07-17 13:27:00 -07:00
ipc Merge branch 'akpm' (patches from Andrew) 2021-07-02 12:08:10 -07:00
kernel workqueue: Assign a color to barrier work items 2021-08-17 07:49:10 -10:00
lib lib/test_hmm: remove set but unused page variable 2021-07-15 10:13:49 -07:00
LICENSES LICENSES/dual/CC-BY-4.0: Git rid of "smart quotes" 2021-07-15 06:31:24 -06:00
mm mm: fix the deadlock in finish_fault() 2021-07-23 17:43:28 -07:00
net af_unix: fix garbage collect vs MSG_PEEK 2021-07-28 10:18:00 -07:00
samples Networking fixes for 5.14-rc2, including fixes from bpf and netfilter. 2021-07-14 09:24:32 -07:00
scripts Kbuild fixes for v5.14 2021-07-18 11:10:30 -07:00
security asm-generic/unaligned: Unify asm/unaligned.h around struct helper 2021-07-02 12:43:40 -07:00
sound sound fixes for 5.14-rc3 2021-07-23 09:58:23 -07:00
tools ARM: 2021-07-29 09:42:09 -07:00
usr .gitignore: prefix local generated files with a slash 2021-05-02 00:43:35 +09:00
virt KVM: add missing compat KVM_CLEAR_DIRTY_LOG 2021-07-27 16:59:01 -04:00
.clang-format clang-format: Update with the latest for_each macro list 2021-05-12 23:32:39 +02:00
.cocciconfig
.get_maintainer.ignore Opt out of scripts/get_maintainer.pl 2019-05-16 10:53:40 -07:00
.gitattributes .gitattributes: use 'dts' diff driver for dts files 2019-12-04 19:44:11 -08:00
.gitignore .gitignore: ignore only top-level modules.builtin 2021-05-02 00:43:35 +09:00
.mailmap m68k updates for v5.14 2021-06-28 14:01:03 -07:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS MAINTAINERS: move Murali Karicheri to credits 2021-04-29 15:47:30 -07:00
Kbuild kbuild: rename hostprogs-y/always to hostprogs/always-y 2020-02-04 01:53:07 +09:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS Char/Misc fixes for 5.14-rc3 2021-07-23 10:14:56 -07:00
Makefile Linux 5.14-rc3 2021-07-25 15:35:14 -07:00
README Drop all 00-INDEX files from Documentation/ 2018-09-09 15:08:58 -06:00

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.