On 07/24/2013 09:55 PM, Tejun Heo wrote:
> 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 <la...@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 <la...@cn.fujitsu.com>
> Reported-by: "Srivatsa S. Bhat" <srivatsa.b...@linux.vnet.ibm.com>
> Reported-by: Alexander Duyck <alexander.h.du...@intel.com>
> Signed-off-by: Tejun Heo <t...@kernel.org>
> ---
This version works as well, it fixes the issue I was facing.
Thank you!
FWIW:
Tested-by: Srivatsa S. Bhat <srivatsa.b...@linux.vnet.ibm.com>
Regards,
Srivatsa S. Bhat
> 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);
>
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