[PATCH] [PATCH] workqueue: allow the function of work_on_cpu() cancall work_on_cpu()

[Lai Jiangshan]

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,
But we don't want to change the flush_work(), so we use
completion instead of flush_work() in the work_on_cpu().

Reported-by: Srivatsa S. Bhat <srivatsa.bhat@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxx>
---
 kernel/workqueue.c |    5 ++++-
 1 files changed, 4 insertions(+), 1 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index f02c4a4..b021a45 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -4731,6 +4731,7 @@ struct work_for_cpu {
 	long (*fn)(void *);
 	void *arg;
 	long ret;
+	struct completion done;
 };
 
 static void work_for_cpu_fn(struct work_struct *work)
@@ -4738,6 +4739,7 @@ static void work_for_cpu_fn(struct work_struct *work)
 	struct work_for_cpu *wfc = container_of(work, struct work_for_cpu, work);
 
 	wfc->ret = wfc->fn(wfc->arg);
+	complete(&wfc->done);
 }
 
 /**
@@ -4755,8 +4757,9 @@ long work_on_cpu(int cpu, long (*fn)(void *), void *arg)
 	struct work_for_cpu wfc = { .fn = fn, .arg = arg };
 
 	INIT_WORK_ONSTACK(&wfc.work, work_for_cpu_fn);
+	init_completion(&wfc.done);
 	schedule_work_on(cpu, &wfc.work);
-	flush_work(&wfc.work);
+	wait_for_completion(&wfc.done);
 	return wfc.ret;
 }
 EXPORT_SYMBOL_GPL(work_on_cpu);
-- 
1.7.4.4
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