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Message-ID: <bc5743cb-f79e-2f67-d594-85b56f05bda3@fujitsu.com>
Date:   Wed, 15 Dec 2021 10:06:29 +0800
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Subject: Re: [PATCH v8 1/9] dax: Use percpu rwsem for dax_{read,write}_lock()
To:     Christoph Hellwig <hch@infradead.org>
CC:     <linux-kernel@vger.kernel.org>, <linux-xfs@vger.kernel.org>,
        <nvdimm@lists.linux.dev>, <linux-mm@kvack.org>,
        <linux-fsdevel@vger.kernel.org>, <djwong@kernel.org>,
        <dan.j.williams@intel.com>, <david@fromorbit.com>,
        <jane.chu@oracle.com>
References: <20211202084856.1285285-1-ruansy.fnst@fujitsu.com>
 <20211202084856.1285285-2-ruansy.fnst@fujitsu.com>
 <Ybi69MCK5sP4ebwG@infradead.org>
From:   Shiyang Ruan <ruansy.fnst@fujitsu.com>
In-Reply-To: <Ybi69MCK5sP4ebwG@infradead.org>
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在 2021/12/14 23:40, Christoph Hellwig 写道:
> On Thu, Dec 02, 2021 at 04:48:48PM +0800, Shiyang Ruan wrote:
>> In order to introduce dax holder registration, we need a write lock for
>> dax.  Change the current lock to percpu_rw_semaphore and introduce a
>> write lock for registration.
> 
> Why do we need to change the existing, global locking for that?

I think we have talked about this in the previous v7 patchset:

 
https://lore.kernel.org/nvdimm/20210924130959.2695749-1-ruansy.fnst@fujitsu.com/T/#m4031bc3dc49dcbaac6f8d99877f910fa9a6f998a

If it is a global lock, any write lock will block other dax devices.

> 
> What is the impact of this to benchmarks?  Also if we stop using srcu
> protection, we should be able to get rid of grace periods or RCU frees.

I didn't test in benchmarks for now.  Could you show me which one I 
should test this code on?  I am not familiar with this...


--
Thanks,
Ruan.