Date:	Mon, 23 Mar 2009 21:23:04 -0600
From:	Alex Chiang <achiang@...com>
To:	mingo@...e.hu
Cc:	jbarnes@...tuousgeek.org, linux-pci@...r.kernel.org,
	linux-kernel@...r.kernel.org, kaneshige.kenji@...fujitsu.com
Subject: Re: [PATCH v5 09/13] PCI: Introduce /sys/bus/pci/devices/.../remove

Hi Ingo,

* Kenji Kaneshige <kaneshige.kenji@...fujitsu.com>:
> Alex Chiang wrote:
>> This patch adds an attribute named "remove" to a PCI device's sysfs
>> directory.  Writing a non-zero value to this attribute will remove the PCI
>> device and any children of it.
>>
>> Trent Piepho wrote the original implementation and documentation.
>>
>> Thanks to Vegard Nossum for testing under kmemcheck and finding locking
>> issues with the sysfs interface.
>>
>> Cc: Trent Piepho <xyzzy@...akeasy.org>
>> Signed-off-by: Alex Chiang <achiang@...com>

[snip part of patch]

>>  diff --git a/drivers/pci/pci-sysfs.c b/drivers/pci/pci-sysfs.c
>> index be7468a..e16990e 100644
>> --- a/drivers/pci/pci-sysfs.c
>> +++ b/drivers/pci/pci-sysfs.c
>> @@ -243,6 +243,39 @@ struct bus_attribute pci_bus_attrs[] = {
>>  	__ATTR(rescan, (S_IWUSR|S_IWGRP), NULL, bus_rescan_store),
>>  	__ATTR_NULL
>>  };
>> +
>> +static void remove_callback(struct device *dev)
>> +{
>> +	struct pci_dev *pdev = to_pci_dev(dev);
>> +
>> +	mutex_lock(&pci_remove_rescan_mutex);
>> +	pci_remove_bus_device(pdev);
>> +	mutex_unlock(&pci_remove_rescan_mutex);
>> +}
>> +
>> +static ssize_t
>> +remove_store(struct device *dev, struct device_attribute *dummy,
>> +	     const char *buf, size_t count)
>> +{
>> +	int ret = 0;
>> +	unsigned long val;
>> +	struct pci_dev *pdev = to_pci_dev(dev);
>> +
>> +	if (strict_strtoul(buf, 0, &val) < 0)
>> +		return -EINVAL;
>> +
>> +	if (pci_is_root_bus(pdev->bus))
>> +		return -EBUSY;
>> +
>> +	/* An attribute cannot be unregistered by one of its own methods,
>> +	 * so we have to use this roundabout approach.
>> +	 */
>> +	if (val)
>> +		ret = device_schedule_callback(dev, remove_callback);
>> +	if (ret)
>> +		count = ret;
>> +	return count;
>> +}
>>  #endif
>>  

Kenji Kaneshige reported the below lockdep problem when testing
my patch on one of his machines.

> I still have the following kernel error messages in testing with your
> latest set of patches (Jesse's linux-next). The test case is removing
> e1000e device or its parent bridge by "echo 1 > /sys/bus/pci/devices/
> .../remove".
>
> [  537.379995] =============================================
> [  537.380124] [ INFO: possible recursive locking detected ]
> [  537.380128] 2.6.29-rc8-kk #1
> [  537.380128] ---------------------------------------------
> [  537.380128] events/4/56 is trying to acquire lock:
> [  537.380128]  (events){--..}, at: [<ffffffff80257fc0>] flush_workqueue+0x0/0xa0
> [  537.380128]
> [  537.380128] but task is already holding lock:
> [  537.380128]  (events){--..}, at: [<ffffffff80257648>] run_workqueue+0x108/0x230
> [  537.380128]
> [  537.380128] other info that might help us debug this:
> [  537.380128] 3 locks held by events/4/56:
> [  537.380128]  #0:  (events){--..}, at: [<ffffffff80257648>] run_workqueue+0x108/0x230
> [  537.380128]  #1:  (&ss->work){--..}, at: [<ffffffff80257648>] run_workqueue+0x108/0x230
> [  537.380128]  #2:  (pci_remove_rescan_mutex){--..}, at: [<ffffffff803c10d1>] remove_callback+0x21/0x40
> [  537.380128]
> [  537.380128] stack backtrace:
> [  537.380128] Pid: 56, comm: events/4 Not tainted 2.6.29-rc8-kk #1
> [  537.380128] Call Trace:
> [  537.380128]  [<ffffffff8026dfcd>] validate_chain+0xb7d/0x1260
> [  537.380128]  [<ffffffff8026eade>] __lock_acquire+0x42e/0xa40
> [  537.380128]  [<ffffffff8026f148>] lock_acquire+0x58/0x80
> [  537.380128]  [<ffffffff80257fc0>] ? flush_workqueue+0x0/0xa0
> [  537.380128]  [<ffffffff8025800d>] flush_workqueue+0x4d/0xa0
> [  537.380128]  [<ffffffff80257fc0>] ? flush_workqueue+0x0/0xa0
> [  537.383380]  [<ffffffff80258070>] flush_scheduled_work+0x10/0x20
> [  537.383380]  [<ffffffffa0144065>] e1000_remove+0x55/0xfe [e1000e]
> [  537.383380]  [<ffffffff8033ee30>] ? sysfs_schedule_callback_work+0x0/0x50
> [  537.383380]  [<ffffffff803bfeb2>] pci_device_remove+0x32/0x70
> [  537.383380]  [<ffffffff80441da9>] __device_release_driver+0x59/0x90
> [  537.383380]  [<ffffffff80441edb>] device_release_driver+0x2b/0x40
> [  537.383380]  [<ffffffff804419d6>] bus_remove_device+0xa6/0x120
> [  537.384382]  [<ffffffff8043e46b>] device_del+0x12b/0x190
> [  537.384382]  [<ffffffff8043e4f6>] device_unregister+0x26/0x70
> [  537.384382]  [<ffffffff803ba969>] pci_stop_dev+0x49/0x60
> [  537.384382]  [<ffffffff803baab0>] pci_remove_bus_device+0x40/0xc0
> [  537.384382]  [<ffffffff803c10d9>] remove_callback+0x29/0x40
> [  537.384382]  [<ffffffff8033ee4f>] sysfs_schedule_callback_work+0x1f/0x50
> [  537.384382]  [<ffffffff8025769a>] run_workqueue+0x15a/0x230
> [  537.384382]  [<ffffffff80257648>] ? run_workqueue+0x108/0x230
> [  537.384382]  [<ffffffff8025846f>] worker_thread+0x9f/0x100
> [  537.384382]  [<ffffffff8025bce0>] ? autoremove_wake_function+0x0/0x40
> [  537.384382]  [<ffffffff802583d0>] ? worker_thread+0x0/0x100
> [  537.384382]  [<ffffffff8025b89d>] kthread+0x4d/0x80
> [  537.384382]  [<ffffffff8020d4ba>] child_rip+0xa/0x20
> [  537.386380]  [<ffffffff8020cebc>] ? restore_args+0x0/0x30
> [  537.386380]  [<ffffffff8025b850>] ? kthread+0x0/0x80
> [  537.386380]  [<ffffffff8020d4b0>] ? child_rip+0x0/0x20
>
> I think the cause of this error message is flush_workqueue()
> from the work of keventd. When removing device using
> "/sys/bus/pci/devices/.../ remove", pci_remove_bus_device() is
> executed by the keventd's work through
> device_schedule_callback(), and it invokes e1000e's remove
> callback. And then, e1000e's remove callback invokes
> flush_workqueue().  Actually, the kernel error messages are not
> displayed when I changed e1000e driver to not call
> flush_workqueue(). In my understanding, flush_workqueue() from
> the work must be avoided because it can cause a deadlock.
> Please note that this is not a problem of e1000e driver.
> Drivers can use flush_workqueue(), of course.

I agree with this analysis; the reason we're seeing this lockdep
warning is because the sysfs attributed scheduled a removal for
itself using device_schedule_callback(). This is necessary
because sysfs attributes can't remove themselves due to other
locking issues.

My question is -- is it a bug to call flush_workqueue during
run_workqueue?

Conceptually, I don't think it should be a bug; it should be a
nop, since run_workqueue _is_ flushing the work queue.

Thoughts?

> BTW, I also have another worry about executing pci_remove_bus_device()
> by the work of keventd. The pci_remove_bus_device() will take a long
> time  especially when the bridge device near the root bus is specified.
> The long delay of keventd's work will have bad effects to other works
> on the workqueue.

The real fix is to fix sysfs so that attributes can remove
themselves directly. I will work with Tejun Heo on getting this
working sooner rather than later. That will avoid the locking
issue you discovered above as well as the concern you point out
about putting long running tasks in the keventd work queue.

Thanks.

/ac

>
> Thanks,
> Kenji Kaneshige
>
>
>
>>  struct device_attribute pci_dev_attrs[] = {
>> @@ -263,6 +296,9 @@ struct device_attribute pci_dev_attrs[] = {
>>  	__ATTR(broken_parity_status,(S_IRUGO|S_IWUSR),
>>  		broken_parity_status_show,broken_parity_status_store),
>>  	__ATTR(msi_bus, 0644, msi_bus_show, msi_bus_store),
>> +#ifdef CONFIG_HOTPLUG
>> +	__ATTR(remove, (S_IWUSR|S_IWGRP), NULL, remove_store),
>> +#endif
>>  	__ATTR_NULL,
>>  };
>>  
>>
>> --
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>>
>
>
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