Commit c2fda509 authored Jul 24, 2013 by Lai Jiangshan Committed by Tejun Heo Jul 24, 2013

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 <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>

parent ad81f054

kernel/workqueue.c

+22 −10

Original line number	Diff line number	Diff line
		@@ -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);