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

[wangyijing <wangyijing@huawei.com>]

> 
> 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);
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.

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

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