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workqueue: Wrap flush_workqueue() using a macro

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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@I-love.SAKURA.ne.jp>
Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Tetsuo Handa
2022-06-01 16:32:47 +09:00
committed by Tejun Heo
parent e71e60cd74
commit c4f135d643
2 changed files with 68 additions and 12 deletions
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kernel/workqueue.c
+12 -4
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@@ -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);