Re: [RFC PATCH] scsi: libsas: fix WARN on device removal

From: John Garry <john.garry@huawei.com>
To: Dan Williams <dan.j.williams@intel.com>
Cc: "Martin K. Petersen" <martin.petersen@oracle.com>,
	<jejb@linux.vnet.ibm.com>,
	linux-scsi <linux-scsi@vger.kernel.org>,
	<john.garry2@mail.dcu.ie>, <linuxarm@huawei.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	<lindar_liu@usish.com>, <jinpu.wang@profitbricks.com>,
	Tejun Heo <tj@kernel.org>
Subject: Re: [RFC PATCH] scsi: libsas: fix WARN on device removal
Date: Thu, 10 Nov 2016 11:53:14 +0000	[thread overview]
Message-ID: <9870e7bc-a472-1913-1930-ac022e8ad5e8@huawei.com> (raw)
In-Reply-To: <CAPcyv4hvuFLhS2sYXkccXMTSh-ggxo7uHN2Qt6i5g8GkyBksrw@mail.gmail.com>

On 09/11/2016 20:35, Dan Williams wrote:
> On Wed, Nov 9, 2016 at 11:09 AM, Dan Williams <dan.j.williams@intel.com> wrote:
>> On Wed, Nov 9, 2016 at 9:36 AM, John Garry <john.garry@huawei.com> wrote:
>>> On 09/11/2016 12:28, John Garry wrote:
>>>>
>>>> On 03/11/2016 14:58, John Garry wrote:
>>>>>
>>>>> The following patch introduces an annoying WARN
>>>>> when a device is removed from the SAS topology:
>>>>> [SCSI] libsas: prevent domain rediscovery competing with ata error
>>>>> handling
>>>>>
>>>>
>>>> Are there any views on this patch? I would have thought that the parties
>>>> who use the drivers based on libsas would be interested in fixing this
>>>> bug.
>>>>
>>>
>>> I should have added the before and after logs earlier, so the issue is
>>> illustrated. Now attached. When a 24-port expander is unplugged we get >6k
>>> lines of WARN on the console, lasting >30 seconds. Not nice.
>>>
>>
>> I might be mistaken, but this patch seems functionally identical to
>> this attempt:
>>
>> http://marc.info/?l=linux-scsi&m=143459794823595&w=2

Hi Dan,

They're not the same. I don't see how your solution properly deals with 
remote sas_port deletion.

When we unplug a device connected to an expander, can't the sas_port be 
deleted twice, in sas_unregister_devs_sas_addr() from domain 
revalidation and also now in sas_destruct_devices()? I think that this 
gives a NULL dereference.
And we still get the WARN as the sas_port has still been deleted before 
the device.

In my solution, we should always delete the sas_port after the attached 
device.

>>
>> i.e. it moves the port destruction to the workqueue and still suffers
>> from the flutter problem:
>>
>> http://marc.info/?l=linux-scsi&m=143801026028006&w=2
>> http://marc.info/?l=linux-scsi&m=143801971131073&w=2
>>
>> Perhaps we instead need to quiet this warning?
>>
>> http://marc.info/?l=linux-scsi&m=143802229932175&w=2

I have not seen the flutter issue. I am just trying to solve the 
horrible WARN dump.
However I do understand that there may be a issue related to how we 
queue the events; there was a recent attempt to fix this, but it came to 
nothing:
https://www.spinics.net/lists/linux-scsi/msg99991.html

Cheers,
John

>
> Alternatively we need a mechanism to cancel in-flight port shutdown
> requests when we start re-attaching devices before queued port
> destruction events have run.
>
> .
>