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

From: John Garry <john.garry@huawei.com>
To: wangyijing <wangyijing@huawei.com>,
	Dan Williams <dan.j.williams@intel.com>
Cc: <jejb@linux.vnet.ibm.com>,
	"Martin K. Petersen" <martin.petersen@oracle.com>,
	linux-scsi <linux-scsi@vger.kernel.org>,
	<john.garry2@mail.dcu.ie>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	<linuxarm@huawei.com>, <lindar_liu@usish.com>,
	Tejun Heo <tj@kernel.org>, <jinpu.wang@profitbricks.com>
Subject: Re: [RFC PATCH] scsi: libsas: fix WARN on device removal
Date: Fri, 11 Nov 2016 08:23:40 +0000	[thread overview]
Message-ID: <93ae84f6-75a2-f576-808e-f98c6256b6a6@huawei.com> (raw)
In-Reply-To: <58257D52.6090507@huawei.com>

On 11/11/2016 08:12, wangyijing wrote:
>>
>> 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
>
> We found libsas hotplug several problems:
> 1. sysfs warning calltrace(like the case you found);

Maybe you can then review my patch.

> 2. hot-add and hot-remove work events may process out of order;
> 3. in some extreme cases, libsas may miss some events, if the same event is still pending in workqueue.
>

Can you tell me how to recreate #2 and #3?

> It's a complex issue, we posted two patches, try to fix these issues, but now few people are interested in it  :(
>

IIRC, you sent as RFC and got a "reviewed-by" from Hannes, so I'm not 
sure what else you want. BTW, I thought that the changes were quite drastic.

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.
>>>
>>> .
>>>
>>
>>
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