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Message-ID: <f8c281cd-6ce7-818d-8c43-6024a7b3b481@I-love.SAKURA.ne.jp>
Date: Wed, 1 Jun 2022 16:32:47 +0900
From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To: Tejun Heo <tj@...nel.org>,
Andrew Morton <akpm@...ux-foundation.org>
Cc: LKML <linux-kernel@...r.kernel.org>
Subject: [PATCH v5] workqueue: Wrap flush_workqueue() using a macro
Since flush operation synchronously waits for completion, flushing
system-wide WQs (e.g. system_wq) might introduce possibility of deadlock
due to unexpected locking dependency. Tejun Heo commented at [1] 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_wq WQ [2].
Therefore, let's change the direction to that developers had better use
their local WQs if flush_scheduled_work()/flush_workqueue(system_*_wq) is
inevitable.
Steps for converting system-wide WQs into local WQs are explained at [3],
and a conversion to stop flushing system-wide WQs is in progress. Now we
want some mechanism for preventing developers who are not aware of this
conversion from again start flushing system-wide WQs.
Since I found that WARN_ON() is complete but awkward approach for teaching
developers about this problem, let's use __compiletime_warning() for
incomplete but handy approach. For completeness, we will also insert
WARN_ON() into __flush_workqueue() after all in-tree users stopped calling
flush_scheduled_work().
Link: https://lore.kernel.org/all/YgnQGZWT%2Fn3VAITX@slm.duckdns.org/ [1]
Link: https://syzkaller.appspot.com/bug?extid=bde0f89deacca7c765b8 [2]
Link: https://lkml.kernel.org/r/49925af7-78a8-a3dd-bce6-cfc02e1a9236@I-love.SAKURA.ne.jp [3]
Signed-off-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
---
Changes in v5:
Revert use of inline function, for it turned out that clang emits warning with
the line of where flush_workqueue() being defined (i.e. some fixed value in
include/linux/workqueue.h) rather than where flush_workqueue() being called
if flush_workqueue() is defined as an inline function. We need to use a macro
in order to report where flush_workqueue() is called. (Since frontend became
a macro, we might no longer need to rename backend to __flush_workqueue()...)
Add comment about why not to emit warning from flush_scheduled_work()
and why not to use BUILD_BUG_ON() from flush_workqueue().
v4 is at https://lkml.kernel.org/r/f9743e46-d467-82aa-effc-597b38e7377d@I-love.SAKURA.ne.jp
include/linux/workqueue.h | 64 ++++++++++++++++++++++++++++++++++-----
kernel/workqueue.c | 16 +++++++---
2 files changed, 68 insertions(+), 12 deletions(-)
diff --git a/include/linux/workqueue.h b/include/linux/workqueue.h
index 7fee9b6cfede..e1f1c8b1121b 100644
--- a/include/linux/workqueue.h
+++ b/include/linux/workqueue.h
@@ -445,7 +445,7 @@ extern bool mod_delayed_work_on(int cpu, struct workqueue_struct *wq,
struct delayed_work *dwork, unsigned long delay);
extern bool queue_rcu_work(struct workqueue_struct *wq, struct rcu_work *rwork);
-extern void flush_workqueue(struct workqueue_struct *wq);
+extern void __flush_workqueue(struct workqueue_struct *wq);
extern void drain_workqueue(struct workqueue_struct *wq);
extern int schedule_on_each_cpu(work_func_t func);
@@ -563,15 +563,23 @@ static inline bool schedule_work(struct work_struct *work)
return queue_work(system_wq, work);
}
+/*
+ * Detect attempt to flush system-wide workqueues at compile time when possible.
+ *
+ * See https://lkml.kernel.org/r/49925af7-78a8-a3dd-bce6-cfc02e1a9236@I-love.SAKURA.ne.jp
+ * for reasons and steps for converting system-wide workqueues into local workqueues.
+ */
+extern void __warn_flushing_systemwide_wq(void)
+ __compiletime_warning("Please avoid flushing system-wide workqueues.");
+
/**
* flush_scheduled_work - ensure that any scheduled work has run to completion.
*
* 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:
+ * 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.
@@ -586,11 +594,51 @@ static inline bool schedule_work(struct work_struct *work)
* need to know that a particular work item isn't queued and isn't running.
* In such cases you should use cancel_delayed_work_sync() or
* cancel_work_sync() instead.
+ *
+ * Please stop calling this function! A conversion to stop flushing system-wide
+ * workqueues is in progress. This function will be removed after all in-tree
+ * users stopped calling this function.
*/
-static inline void flush_scheduled_work(void)
-{
- flush_workqueue(system_wq);
-}
+/*
+ * The background of commit 771c035372a036f8 ("deprecate the
+ * '__deprecated' attribute warnings entirely and for good") is that,
+ * since Linus builds all modules between every single pull he does,
+ * the standard kernel build needs to be _clean_ in order to be able to
+ * notice when new problems happen. Therefore, don't emit warning while
+ * there are in-tree users.
+ */
+#define flush_scheduled_work() \
+({ \
+ if (0) \
+ __warn_flushing_systemwide_wq(); \
+ __flush_workqueue(system_wq); \
+})
+
+/*
+ * Although there is no longer in-tree caller, for now just emit warning
+ * in order to give out-of-tree callers time to update.
+ */
+#define flush_workqueue(wq) \
+({ \
+ struct workqueue_struct *_wq = (wq); \
+ \
+ if ((__builtin_constant_p(_wq == system_wq) && \
+ _wq == system_wq) || \
+ (__builtin_constant_p(_wq == system_highpri_wq) && \
+ _wq == system_highpri_wq) || \
+ (__builtin_constant_p(_wq == system_long_wq) && \
+ _wq == system_long_wq) || \
+ (__builtin_constant_p(_wq == system_unbound_wq) && \
+ _wq == system_unbound_wq) || \
+ (__builtin_constant_p(_wq == system_freezable_wq) && \
+ _wq == system_freezable_wq) || \
+ (__builtin_constant_p(_wq == system_power_efficient_wq) && \
+ _wq == system_power_efficient_wq) || \
+ (__builtin_constant_p(_wq == system_freezable_power_efficient_wq) && \
+ _wq == system_freezable_power_efficient_wq)) \
+ __warn_flushing_systemwide_wq(); \
+ __flush_workqueue(_wq); \
+})
/**
* schedule_delayed_work_on - queue work in global workqueue on CPU after delay
diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 4056f2a3f9d5..1ea50f6be843 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -2788,13 +2788,13 @@ static bool flush_workqueue_prep_pwqs(struct workqueue_struct *wq,
}
/**
- * flush_workqueue - ensure that any scheduled work has run to completion.
+ * __flush_workqueue - ensure that any scheduled work has run to completion.
* @wq: workqueue to flush
*
* This function sleeps until all work items which were queued on entry
* have finished execution, but it is not livelocked by new incoming ones.
*/
-void flush_workqueue(struct workqueue_struct *wq)
+void __flush_workqueue(struct workqueue_struct *wq)
{
struct wq_flusher this_flusher = {
.list = LIST_HEAD_INIT(this_flusher.list),
@@ -2943,7 +2943,7 @@ void flush_workqueue(struct workqueue_struct *wq)
out_unlock:
mutex_unlock(&wq->mutex);
}
-EXPORT_SYMBOL(flush_workqueue);
+EXPORT_SYMBOL(__flush_workqueue);
/**
* drain_workqueue - drain a workqueue
@@ -2971,7 +2971,7 @@ void drain_workqueue(struct workqueue_struct *wq)
wq->flags |= __WQ_DRAINING;
mutex_unlock(&wq->mutex);
reflush:
- flush_workqueue(wq);
+ __flush_workqueue(wq);
mutex_lock(&wq->mutex);
@@ -6111,3 +6111,11 @@ void __init workqueue_init(void)
wq_online = true;
wq_watchdog_init();
}
+
+/*
+ * Despite the naming, this is a no-op function which is here only for avoiding
+ * link error. Since compile-time warning may fail to catch, we will need to
+ * emit run-time warning from __flush_workqueue().
+ */
+void __warn_flushing_systemwide_wq(void) { }
+EXPORT_SYMBOL(__warn_flushing_systemwide_wq);
--
2.18.4
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