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Subject[PATCH] workqueue: allow work_on_cpu() to be called recursively
Applied to wq/for-3.11-fixes with comment and subject tweaks.

Thanks!

---------- 8< ------------

From c2fda509667b0fda4372a237f5a59ea4570b1627 Mon Sep 17 00:00:00 2001
From: Lai Jiangshan <laijs@cn.fujitsu.com>
Date: Wed, 24 Jul 2013 18:31:42 +0800

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.

tj: Minor comment adjustment.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Reported-by: "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>
Reported-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
---
kernel/workqueue.c | 32 ++++++++++++++++++++++----------
1 file changed, 22 insertions(+), 10 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index f02c4a4..55f5f0a 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 +4761,14 @@ 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);
+
+ /*
+ * The work item is on-stack and can't lead to deadlock through
+ * flushing. Use __flush_work() to avoid spurious lockdep warnings
+ * when work_on_cpu()s are nested.
+ */
+ __flush_work(&wfc.work);
+
return wfc.ret;
}
EXPORT_SYMBOL_GPL(work_on_cpu);
--
1.8.3.1


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