workqueue: allow work_on_cpu() to be called recursively
workqueue: allow work_on_cpu() to be called recursively

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 <4c9eb49378914b743bdd32292c04df29b820d73e@cn.fujitsu.com>
Reported-by: "Srivatsa S. Bhat" <5b23f8226e886b22344fc642373ea657dd596b63@linux.vnet.ibm.com>
Reported-by: Alexander Duyck <af56634f9be8457194a3461ec96c497f138af4ff@intel.com>
Signed-off-by: Tejun Heo <546b05909706652891a87f7bfe385ae147f61f91@kernel.org>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs