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Date:	Thu, 18 Apr 2013 15:57:36 -0600
From:	Bjorn Helgaas <bhelgaas@...gle.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Or Gerlitz <ogerlitz@...lanox.com>,
	Ming Lei <ming.lei@...onical.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	David Miller <davem@...emloft.net>,
	Roland Dreier <roland@...nel.org>,
	netdev <netdev@...r.kernel.org>, Yan Burman <yanb@...lanox.com>,
	Jack Morgenstein <jackm@....mellanox.co.il>,
	Tejun Heo <tj@...nel.org>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>
Subject: Re: [PATCHv2 for-3.9] pci: avoid work_on_cpu for nested SRIOV probes

On Thu, Apr 18, 2013 at 3:40 PM, Bjorn Helgaas <bhelgaas@...gle.com> wrote:
> On Thu, Apr 18, 2013 at 2:08 PM, Michael S. Tsirkin <mst@...hat.com> wrote:
>> The following lockdep report triggers since 3.9-rc1:
>>
>> =============================================
>> [ INFO: possible recursive locking detected ]
>> 3.9.0-rc1 #96 Not tainted
>> ---------------------------------------------
>> kworker/0:1/734 is trying to acquire lock:
>>  ((&wfc.work)){+.+.+.}, at: [<ffffffff81066cb0>] flush_work+0x0/0x250
>>
>> but task is already holding lock:
>>  ((&wfc.work)){+.+.+.}, at: [<ffffffff81064352>]
>> process_one_work+0x162/0x4c0
>>
>> other info that might help us debug this:
>>  Possible unsafe locking scenario:
>>
>>        CPU0
>>        ----
>>   lock((&wfc.work));
>>   lock((&wfc.work));
>>
>>  *** DEADLOCK ***
>>
>>  May be due to missing lock nesting notation
>>
>> 3 locks held by kworker/0:1/734:
>>  #0:  (events){.+.+.+}, at: [<ffffffff81064352>]
>> process_one_work+0x162/0x4c0
>>  #1:  ((&wfc.work)){+.+.+.}, at: [<ffffffff81064352>]
>> process_one_work+0x162/0x4c0
>>  #2:  (&__lockdep_no_validate__){......}, at: [<ffffffff812db225>]
>> device_attach+0x25/0xb0
>>
>> stack backtrace:
>> Pid: 734, comm: kworker/0:1 Not tainted 3.9.0-rc1 #96
>> Call Trace:
>>  [<ffffffff810948ec>] validate_chain+0xdcc/0x11f0
>>  [<ffffffff81095150>] __lock_acquire+0x440/0xc70
>>  [<ffffffff81095150>] ? __lock_acquire+0x440/0xc70
>>  [<ffffffff810959da>] lock_acquire+0x5a/0x70
>>  [<ffffffff81066cb0>] ? wq_worker_waking_up+0x60/0x60
>>  [<ffffffff81066cf5>] flush_work+0x45/0x250
>>  [<ffffffff81066cb0>] ? wq_worker_waking_up+0x60/0x60
>>  [<ffffffff810922be>] ? mark_held_locks+0x9e/0x130
>>  [<ffffffff81066a96>] ? queue_work_on+0x46/0x90
>>  [<ffffffff810925dd>] ? trace_hardirqs_on_caller+0xfd/0x190
>>  [<ffffffff8109267d>] ? trace_hardirqs_on+0xd/0x10
>>  [<ffffffff81066f74>] work_on_cpu+0x74/0x90
>>  [<ffffffff81063820>] ? keventd_up+0x20/0x20
>>  [<ffffffff8121fd30>] ? pci_pm_prepare+0x60/0x60
>>  [<ffffffff811f9293>] ? cpumask_next_and+0x23/0x40
>>  [<ffffffff81220a1a>] pci_device_probe+0xba/0x110
>>  [<ffffffff812dadca>] ? driver_sysfs_add+0x7a/0xb0
>>  [<ffffffff812daf1f>] driver_probe_device+0x8f/0x230
>>  [<ffffffff812db170>] ? __driver_attach+0xb0/0xb0
>>  [<ffffffff812db1bb>] __device_attach+0x4b/0x60
>>  [<ffffffff812d9314>] bus_for_each_drv+0x64/0x90
>>  [<ffffffff812db298>] device_attach+0x98/0xb0
>>  [<ffffffff81218474>] pci_bus_add_device+0x24/0x50
>>  [<ffffffff81232e80>] virtfn_add+0x240/0x3e0
>>  [<ffffffff8146ce3d>] ? _raw_spin_unlock_irqrestore+0x3d/0x80
>>  [<ffffffff812333be>] pci_enable_sriov+0x23e/0x500
>>  [<ffffffffa011fa1a>] __mlx4_init_one+0x5da/0xce0 [mlx4_core]
>>  [<ffffffffa012016d>] mlx4_init_one+0x2d/0x60 [mlx4_core]
>>  [<ffffffff8121fd79>] local_pci_probe+0x49/0x80
>>  [<ffffffff81063833>] work_for_cpu_fn+0x13/0x20
>>  [<ffffffff810643b8>] process_one_work+0x1c8/0x4c0
>>  [<ffffffff81064352>] ? process_one_work+0x162/0x4c0
>>  [<ffffffff81064cfb>] worker_thread+0x30b/0x430
>>  [<ffffffff810649f0>] ? manage_workers+0x340/0x340
>>  [<ffffffff8106cea6>] kthread+0xd6/0xe0
>>  [<ffffffff8106cdd0>] ? __init_kthread_worker+0x70/0x70
>>  [<ffffffff8146daac>] ret_from_fork+0x7c/0xb0
>>  [<ffffffff8106cdd0>] ? __init_kthread_worker+0x70/0x70
>>
>> The issue is that a driver, in it's probe function, calls
>> pci_sriov_enable so a PF device probe causes VF probe (AKA nested
>> probe).  Each probe in pci_device_probe which is (normally) run through
>> work_on_cpu (this is to get the right numa node for memory allocated by
>> the driver).  In turn work_on_cpu does this internally:
>>
>>         schedule_work_on(cpu, &wfc.work);
>>         flush_work(&wfc.work);
>>
>> So if you are running probe on CPU1, and cause another
>> probe on the same CPU, this will try to flush
>> workqueue from inside same workqueue which causes
>> a lockep warning.
>>
>> Nested probing might be tricky to get right generally.
>>
>> But for pci_sriov_enable, the situation is actually very simple: all VFs
>> naturally have same affinity as the PF, and cpumask_any_and is actually
>> same as cpumask_first_and, so it always gives us the same CPU.
>> So let's just detect that, and run the probing for VFs locally without a
>> workqueue.
>>
>> This is hardly elegant, but looks to me like an appropriate quick fix
>> for 3.9.
>>
>> Tested-by: Or Gerlitz <ogerlitz@...lanox.com>
>> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
>> Acked-by: Tejun Heo <tj@...nel.org>
>
> Thanks, Michael.  I put this in my for-linus branch:
>
> http://git.kernel.org/cgit/linux/kernel/git/helgaas/pci.git/log/?h=for-linus
>
> I'll send a pull request to Linus today.

Actually, let me make sure I understand this correctly:  This patch
fixes the lockdep warning, but it does not fix an actual deadlock or
make any functional change.  Is that right?

If that's true, how much pain would it cause to just hold this for
v3.9.1?  I'm nervous about doing a warning fix when we're only a day
or two before releasing v3.9.

Bjorn

>> ---
>>
>> Changes from v1:
>>     - clarified commit log and added Ack by Tejun Heo
>>       patch is unchanged.
>>
>> diff --git a/drivers/pci/pci-driver.c b/drivers/pci/pci-driver.c
>> index 1fa1e48..6eeb5ec 100644
>> --- a/drivers/pci/pci-driver.c
>> +++ b/drivers/pci/pci-driver.c
>> @@ -286,8 +286,8 @@ static int pci_call_probe(struct pci_driver *drv, struct pci_dev *dev,
>>                 int cpu;
>>
>>                 get_online_cpus();
>>                 cpu = cpumask_any_and(cpumask_of_node(node), cpu_online_mask);
>> -               if (cpu < nr_cpu_ids)
>> +               if (cpu != raw_smp_processor_id() && cpu < nr_cpu_ids)
>>                         error = work_on_cpu(cpu, local_pci_probe, &ddi);
>>                 else
>>                         error = local_pci_probe(&ddi);
--
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