lkml.org 
[lkml]   [2022]   [Mar]   [19]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
Date
From
Subject[PATCH v3 (repost)] workqueue: Warn flushing of kernel-global workqueues
Since flush operation synchronously waits for completion, flushing
kernel-global WQs (e.g. system_wq) might introduce possibility of deadlock
due to unexpected locking dependency. Tejun Heo commented that it makes no
sense at all to call flush_workqueue() on the shared WQs as the caller has
no idea what it's gonna end up waiting for.

Although there is flush_scheduled_work() which flushes system_wq WQ with
"Think twice before calling this function! It's very easy to get into
trouble if you don't take great care." warning message, syzbot found a
circular locking dependency caused by flushing system_long_wq WQ [1].
Therefore, let's change the direction to that developers had better use
their local WQs if flush_workqueue() is inevitable.

To give developers time to update their modules, for now just emit
a warning message with ratelimit when flush_workqueue() is called on
kernel-global WQs. We will eventually convert this warning message into
WARN_ON() and kill flush_scheduled_work().

This patch introduces __WQ_NO_FLUSH flag for emitting warning. Don't set
this flag when creating your local WQs while updating your module, for
destroy_workqueue() will involve flush operation.

Theoretically, flushing specific work item using flush_work() queued on
kernel-global WQs (which are !WQ_MEM_RECLAIM) has possibility of deadlock.
But this patch does not emit warning when flush_work() is called on work
items queued on kernel-global WQs, based on assumption that we can create
kworker threads as needed and we won't hit max_active limit.

Link: https://syzkaller.appspot.com/bug?extid=831661966588c802aae9 [1]
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
---
This patch has been in linux-next.git as commit b9c20da356db1b39 ("workqueue:
Warn flushing of kernel-global workqueues") since next-20220301, and I got no
failure reports. I think that this patch is safe to go to linux.git; then
developers will find this patch and update their modules to use their local WQ.

Changes in v3:
Don't check flush_work() attempt.
Use a private WQ_ flag.
Changes in v2:
Removed #ifdef CONFIG_PROVE_LOCKING=y check.
Also check flush_work() attempt.
Shorten warning message.
Introduced a public WQ_ flag, which is initially meant for use by
only system-wide WQs, but allows private WQs used by built-in modules
to use this flag for detecting unexpected flush attempts if they want.

include/linux/workqueue.h | 15 +++------------
kernel/workqueue.c | 36 +++++++++++++++++++++++++++++-------
2 files changed, 32 insertions(+), 19 deletions(-)

diff --git a/include/linux/workqueue.h b/include/linux/workqueue.h
index 7fee9b6cfede..7b13fae377e2 100644
--- a/include/linux/workqueue.h
+++ b/include/linux/workqueue.h
@@ -339,6 +339,7 @@ enum {
__WQ_ORDERED = 1 << 17, /* internal: workqueue is ordered */
__WQ_LEGACY = 1 << 18, /* internal: create*_workqueue() */
__WQ_ORDERED_EXPLICIT = 1 << 19, /* internal: alloc_ordered_workqueue() */
+ __WQ_NO_FLUSH = 1 << 20, /* internal: warn flush_workqueue() */

WQ_MAX_ACTIVE = 512, /* I like 512, better ideas? */
WQ_MAX_UNBOUND_PER_CPU = 4, /* 4 * #cpus for unbound wq */
@@ -569,18 +570,8 @@ static inline bool schedule_work(struct work_struct *work)
* Forces execution of the kernel-global workqueue and blocks until its
* completion.
*
- * Think twice before calling this function! It's very easy to get into
- * trouble if you don't take great care. Either of the following situations
- * will lead to deadlock:
- *
- * One of the work items currently on the workqueue needs to acquire
- * a lock held by your code or its caller.
- *
- * Your code is running in the context of a work routine.
- *
- * They will be detected by lockdep when they occur, but the first might not
- * occur very often. It depends on what work items are on the workqueue and
- * what locks they need, which you have no control over.
+ * Please stop calling this function. If you need to flush kernel-global
+ * workqueue, please use your local workqueue.
*
* In most situations flushing the entire workqueue is overkill; you merely
* need to know that a particular work item isn't queued and isn't running.
diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 33f1106b4f99..bc271579704f 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -2805,6 +2805,25 @@ static bool flush_workqueue_prep_pwqs(struct workqueue_struct *wq,
return wait;
}

+static void warn_flush_attempt(struct workqueue_struct *wq)
+{
+ /*
+ * Since there are known in-tree modules which will emit this warning,
+ * for now don't use WARN_ON() in order not to break kernel testing.
+ *
+ * Print whole traces with ratelimit, in order to make sure that
+ * this warning is not overlooked while this warning does not flood
+ * console and kernel log buffer.
+ */
+ static DEFINE_RATELIMIT_STATE(flush_warn_rs, 600 * HZ, 1);
+
+ ratelimit_set_flags(&flush_warn_rs, RATELIMIT_MSG_ON_RELEASE);
+ if (!__ratelimit(&flush_warn_rs))
+ return;
+ pr_warn("Please do not flush %s WQ.\n", wq->name);
+ dump_stack();
+}
+
/**
* flush_workqueue - ensure that any scheduled work has run to completion.
* @wq: workqueue to flush
@@ -2824,6 +2843,9 @@ void flush_workqueue(struct workqueue_struct *wq)
if (WARN_ON(!wq_online))
return;

+ if (unlikely(wq->flags & __WQ_NO_FLUSH))
+ warn_flush_attempt(wq);
+
lock_map_acquire(&wq->lockdep_map);
lock_map_release(&wq->lockdep_map);

@@ -6054,17 +6076,17 @@ void __init workqueue_init_early(void)
ordered_wq_attrs[i] = attrs;
}

- system_wq = alloc_workqueue("events", 0, 0);
- system_highpri_wq = alloc_workqueue("events_highpri", WQ_HIGHPRI, 0);
- system_long_wq = alloc_workqueue("events_long", 0, 0);
- system_unbound_wq = alloc_workqueue("events_unbound", WQ_UNBOUND,
+ system_wq = alloc_workqueue("events", __WQ_NO_FLUSH, 0);
+ system_highpri_wq = alloc_workqueue("events_highpri", __WQ_NO_FLUSH | WQ_HIGHPRI, 0);
+ system_long_wq = alloc_workqueue("events_long", __WQ_NO_FLUSH, 0);
+ system_unbound_wq = alloc_workqueue("events_unbound", __WQ_NO_FLUSH | WQ_UNBOUND,
WQ_UNBOUND_MAX_ACTIVE);
system_freezable_wq = alloc_workqueue("events_freezable",
- WQ_FREEZABLE, 0);
+ __WQ_NO_FLUSH | WQ_FREEZABLE, 0);
system_power_efficient_wq = alloc_workqueue("events_power_efficient",
- WQ_POWER_EFFICIENT, 0);
+ __WQ_NO_FLUSH | WQ_POWER_EFFICIENT, 0);
system_freezable_power_efficient_wq = alloc_workqueue("events_freezable_power_efficient",
- WQ_FREEZABLE | WQ_POWER_EFFICIENT,
+ __WQ_NO_FLUSH | WQ_FREEZABLE | WQ_POWER_EFFICIENT,
0);
BUG_ON(!system_wq || !system_highpri_wq || !system_long_wq ||
!system_unbound_wq || !system_freezable_wq ||
--
2.32.0

\
 
 \ /
  Last update: 2022-03-19 05:43    [W:0.312 / U:0.072 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site