Re: [PATCH v7 1/8] dax: Use rwsem for dax_{read,write}_lock()

[Christoph Hellwig]

On Fri, Sep 24, 2021 at 09:09:52PM +0800, Shiyang Ruan wrote:
> In order to introduce dax holder registration, we need a write lock for
> dax.  Because of the rarity of notification failures and the infrequency
> of registration events, it would be better to be a global lock rather
> than per-device.  So, change the current lock to rwsem and introduce a
> write lock for registration.

I don't think taking the rw_semaphore everywhere will scale, as
basically any DAX based I/O will take it (in read mode).

So at a minimum we'd need a per-device percpu_rw_semaphore if we want
to go there.