linux-kernel - Re: [PATCH v9 01/10] dax: Use percpu rwsem for dax_{read,write}

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Message-ID: <20220105174517.GI31606@magnolia>
Date:   Wed, 5 Jan 2022 09:45:17 -0800
From:   "Darrick J. Wong" <djwong@...nel.org>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Shiyang Ruan <ruansy.fnst@...itsu.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        Linux NVDIMM <nvdimm@...ts.linux.dev>,
        Linux MM <linux-mm@...ck.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        david <david@...morbit.com>,
        Christoph Hellwig <hch@...radead.org>,
        Jane Chu <jane.chu@...cle.com>
Subject: Re: [PATCH v9 01/10] dax: Use percpu rwsem for
 dax_{read,write}_lock()

On Tue, Jan 04, 2022 at 02:44:08PM -0800, Dan Williams wrote:
> On Sun, Dec 26, 2021 at 6:35 AM Shiyang Ruan <ruansy.fnst@...itsu.com> wrote:
> >
> > In order to introduce dax holder registration, we need a write lock for
> > dax.
> 
> As far as I can see, no, a write lock is not needed while the holder
> is being registered.
> 
> The synchronization that is needed is to make sure that the device
> stays live over the registration event, and that any in-flight holder
> operations are flushed before the device transitions from live to
> dead, and that in turn relates to the live state of the pgmap.
> 
> The dax device cannot switch from live to dead without first flushing
> all readers, so holding dax_read_lock() over the register holder event
> should be sufficient.

...and perhaps add a comment describing that this is what the
synchronization primitive is really protecting against?  The first time
I read through this patchset, I assumed the rwsem was protecting
&dax_hosts and was confused when I saw the one use of dax_write_lock.

--D

> If you are worried about 2 or more potential
> holders colliding at registration time, I would expect that's already
> prevented by block device exclusive holder synchronization, but you
> could also use cmpxchg and a single pointer to a 'struct dax_holder {
> void *holder_data, struct dax_holder_operations *holder_ops }'. If you
> are worried about memory_failure triggering while the filesystem is
> shutting down it can do a synchronize_srcu(&dax_srcu) if it really
> needs to ensure that the notify path is idle after removing the holder
> registration.
> 
> ...are there any cases remaining not covered by the above suggestions?