[PATCH 3/9] nvme: change namespaces_mutext to namespaces_rwsem

[jianchao.wang]

Hi Sagi

Thanks for your kindly response.
And sorry for my bad description. 

On 02/11/2018 07:17 PM, Sagi Grimberg wrote:
>> namespaces_mutext is used to synchronize the operations on ctrl
>> namespaces list. Most of the time, it is a read operation. It is
>> better to change it from mutex to rwsem.
>>
>> On the other hand, the namespaces mutex could introduce circular
>> dependency easily.
> 
> On the other hand of what?
> 
> Also, can you give an example of such?
> 

We have a lot of interface in nvme core that need to hold this 
namespace_mutex and looks like we will have more.
The mutex will trouble us in following scenario.

For example:

context A                        context B
nvme_xxx_xxx                     nvme_xxx_xxx
  -> hold namespace_mutex          -> try to require namespace_mutex
    -> sync context B

Keith's patch will incur this.
http://lists.infradead.org/pipermail/linux-nvme/2018-February/015605.html

Because I add a interface that will hold namespace_mutex, so I add this patchset here.

Thanks
Jianchao
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