On 07/22/2013 02:38 PM, Bjorn Helgaas wrote:
> [+cc Alex, Yinghai, linux-pci]
>
> On Mon, Jul 22, 2013 at 9:37 AM, Srivatsa S. Bhat
> <srivatsa.bhat@xxxxxxxxxxxxxxxxxx> wrote:
>> On 07/22/2013 05:22 PM, Lai Jiangshan wrote:
>>> On 07/19/2013 04:57 PM, Srivatsa S. Bhat wrote:
>>>> On 07/19/2013 07:17 AM, Lai Jiangshan wrote:
>>>>> On 07/19/2013 04:23 AM, Srivatsa S. Bhat wrote:
>>>>>> ---
>>>>>>
>>>>>> kernel/workqueue.c | 6 ++++++
>>>>>> 1 file changed, 6 insertions(+)
>>>>>>
>>>>>>
>>>>>> diff --git a/kernel/workqueue.c b/kernel/workqueue.c
>>>>>> index f02c4a4..07d9a67 100644
>>>>>> --- a/kernel/workqueue.c
>>>>>> +++ b/kernel/workqueue.c
>>>>>> @@ -4754,7 +4754,13 @@ long work_on_cpu(int cpu, long (*fn)(void *), void *arg)
>>>>>> {
>>>>>> struct work_for_cpu wfc = { .fn = fn, .arg = arg };
>>>>>>
>>>>>> +#ifdef CONFIG_LOCKDEP
>>>>>> + static struct lock_class_key __key;
>>>>> Sorry, this "static" should be removed.
>>>>>
>>>> That didn't help either :-( Because it makes lockdep unhappy,
>>>> since the key isn't persistent.
>>>>
>>>> This is the patch I used:
>>>>
>>>> ---
>>>>
>>>> diff --git a/kernel/workqueue.c b/kernel/workqueue.c
>>>> index f02c4a4..7967e3b 100644
>>>> --- a/kernel/workqueue.c
>>>> +++ b/kernel/workqueue.c
>>>> @@ -4754,7 +4754,13 @@ long work_on_cpu(int cpu, long (*fn)(void *), void *arg)
>>>> {
>>>> struct work_for_cpu wfc = { .fn = fn, .arg = arg };
>>>>
>>>> +#ifdef CONFIG_LOCKDEP
>>>> + struct lock_class_key __key;
>>>> + INIT_WORK_ONSTACK(&wfc.work, work_for_cpu_fn);
>>>> + lockdep_init_map(&wfc.work.lockdep_map, "&wfc.work", &__key, 0);
>>>> +#else
>>>> INIT_WORK_ONSTACK(&wfc.work, work_for_cpu_fn);
>>>> +#endif
>>>> schedule_work_on(cpu, &wfc.work);
>>>> flush_work(&wfc.work);
>>>> return wfc.ret;
>>>>
>>>>
>>>> And here are the new warnings:
>>>>
>>>>
>>>> Block layer SCSI generic (bsg) driver version 0.4 loaded (major 252)
>>>> io scheduler noop registered
>>>> io scheduler deadline registered
>>>> io scheduler cfq registered (default)
>>>> BUG: key ffff881039557b98 not in .data!
>>>> ------------[ cut here ]------------
>>>> WARNING: CPU: 8 PID: 1 at kernel/lockdep.c:2987 lockdep_init_map+0x168/0x170()
>>> Sorry again.
>>>
>>> From 0096b9dac2282ec03d59a3f665b92977381a18ad Mon Sep 17 00:00:00 2001
>>> From: Lai Jiangshan <laijs@xxxxxxxxxxxxxx>
>>> Date: Mon, 22 Jul 2013 19:08:51 +0800
>>> Subject: [PATCH] [PATCH] workqueue: allow the function of work_on_cpu() can
>>> call work_on_cpu()
>>>
>>> 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>
>>> ---
>> That worked, thanks a lot!
>>
>> Tested-by: Srivatsa S. Bhat <srivatsa.bhat@xxxxxxxxxxxxxxxxxx>
>>
>> Regards,
>> Srivatsa S. Bhat
>>
>>> 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);
>>>
> Isn't this for the same issue Alex and others have been working on?
>
> It doesn't feel like we have consensus on how this should be fixed.
> You're proposing a change to work_on_cpu(), Alex proposed a change to
> pci_call_probe() [1], Yinghai proposed some changes to the PCI core
> SR-IOV code and several drivers [2].
>
> [1] https://lkml.kernel.org/r/20130624195942.40795.27292.stgit@xxxxxxxxxxxxxxxxxxxxxxxx
> [2] https://lkml.kernel.org/r/1368498506-25857-7-git-send-email-yinghai@xxxxxxxxxx
The solution I proposed was flawed due to possible preemption issues.
If this solution resolves the issue then I am fine with it as long as it
doesn't introduce any new issues.
Thanks,
Alex
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