On 07/23/2013 10:38 PM, Tejun Heo wrote:
> Hey, Lai.
>
> On Tue, Jul 23, 2013 at 09:23:14AM +0800, Lai Jiangshan wrote:
>> The problem is that the userS may not know their work_on_cpu() nested,
>> especially when work_on_cpu()s are on different subsystems and the call depth
>> is deep enough but the nested work_on_cpu() depends on some conditions.
>
> Yeah, that's a possibility. Not sure how much it'd actually matter
> tho given that this is the only instance we have and we've had the
> lockdep annotation for years.
>
>> I prefer to change the user instead of introducing work_on_cpu_nested(), and
>> I accept to change the user only instead of change work_on_cpu() since there
>> is only
>> one nested-calls case found.
>>
>> But I'm thinking, since nested work_on_cpu() don't have any problem,
>> Why workqueue.c don't offer a more friendly API/behavior?
>
> If we wanna solve it from workqueue side, let's please do it by
> introduing an internal flush_work() variant which skips the lockdep
> annotation. I'd really like to avoid using completion here. It's
> nasty as it depends solely on the fact that completion doesn't have
> lockdep annotation yet. Let's do it explicitly.
>
> Thanks.
>
>From 269bf1a2f47f04e0daf429c2cdf4052b4e8fb309 Mon Sep 17 00:00:00 2001
From: Lai Jiangshan <la...@cn.fujitsu.com>
Date: Wed, 24 Jul 2013 18:21:50 +0800
Subject: [PATCH] workqueue: allow the function of work_on_cpu() can call
work_on_cpu()
If the @fn call work_on_cpu() again, the lockdep will complain:
> [ INFO: possible recursive locking detected ]
> 3.11.0-rc1-lockdep-fix-a #6 Not tainted
> ---------------------------------------------
> kworker/0:1/142 is trying to acquire lock:
> ((&wfc.work)){+.+.+.}, at: [<ffffffff81077100>] flush_work+0x0/0xb0
>
> but task is already holding lock:
> ((&wfc.work)){+.+.+.}, at: [<ffffffff81075dd9>] process_one_work+0x169/0x610
>
> other info that might help us debug this:
> Possible unsafe locking scenario:
>
> CPU0
> ----
> lock((&wfc.work));
> lock((&wfc.work));
>
> *** DEADLOCK ***
It is false-positive lockdep report. In this sutiation,
the two "wfc"s of the two work_on_cpu() are different,
they are both on stack. flush_work() can't be deadlock.
To fix this, we need to avoid the lockdep checking in this case,
thus we instroduce a internal __flush_work() which skip the lockdep.
Signed-off-by: Lai Jiangshan <la...@cn.fujitsu.com>
---
kernel/workqueue.c | 29 +++++++++++++++++++----------
1 files changed, 19 insertions(+), 10 deletions(-)
diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index f02c4a4..53df707 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -2817,6 +2817,19 @@ already_gone:
return false;
}
+static bool __flush_work(struct work_struct *work)
+{
+ struct wq_barrier barr;
+
+ if (start_flush_work(work, &barr)) {
+ wait_for_completion(&barr.done);
+ destroy_work_on_stack(&barr.work);
+ return true;
+ } else {
+ return false;
+ }
+}
+
/**
* flush_work - wait for a work to finish executing the last queueing instance
* @work: the work to flush
@@ -2830,18 +2843,10 @@ already_gone:
*/
bool flush_work(struct work_struct *work)
{
- struct wq_barrier barr;
-
lock_map_acquire(&work->lockdep_map);
lock_map_release(&work->lockdep_map);
- if (start_flush_work(work, &barr)) {
- wait_for_completion(&barr.done);
- destroy_work_on_stack(&barr.work);
- return true;
- } else {
- return false;
- }
+ return __flush_work(work);
}
EXPORT_SYMBOL_GPL(flush_work);
@@ -4756,7 +4761,11 @@ long work_on_cpu(int cpu, long (*fn)(void *), void *arg)
INIT_WORK_ONSTACK(&wfc.work, work_for_cpu_fn);
schedule_work_on(cpu, &wfc.work);
- flush_work(&wfc.work);
+ /*
+ * flushing the work can't lead to deadlock, using __flush_work()
+ * to avoid the lockdep complaint for nested work_on_cpu()s.
+ */
+ __flush_work(&wfc.work);
return wfc.ret;
}
EXPORT_SYMBOL_GPL(work_on_cpu);
--
1.7.4.4
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