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Date:   Mon, 2 Oct 2023 19:09:25 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Christoph Hellwig <hch@lst.de>
Cc:     linux-fsdevel@vger.kernel.org,
        Christian Brauner <brauner@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Namjae Jeon <linkinjeon@kernel.org>,
        David Sterba <dsterba@suse.com>,
        David Howells <dhowells@redhat.com>,
        Miklos Szeredi <miklos@szeredi.hu>,
        Amir Goldstein <amir73il@gmail.com>,
        Trond Myklebust <trond.myklebust@hammerspace.com>,
        Bob Peterson <rpeterso@redhat.com>,
        Steve French <sfrench@samba.org>,
        Luis Chamberlain <mcgrof@kernel.org>
Subject: Re: [PATCH 04/15] hfsplus: switch to rcu-delayed unloading of nls
 and freeing ->s_fs_info
Message-ID: <20231002180925.GZ800259@ZenIV>
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On Mon, Oct 02, 2023 at 08:21:43AM +0100, Al Viro wrote:
> On Mon, Oct 02, 2023 at 08:14:01AM +0100, Al Viro wrote:
> > On Mon, Oct 02, 2023 at 08:49:12AM +0200, Christoph Hellwig wrote:
> > > Instead of all this duplicatio in the file system, can we please just
> > > add a
> > > 
> > > 	struct nls_table *s_nls;
> > > 
> > > to struct super_block and RCU free it in common code and drop all the
> > > code in the file systems?
> > 
> > It makes no sense for most of the filesystems, for one thing (note that
> > any use in ->lookup() does not warrant rcu delays).  What's more,
> > how do you formulate the rules for what goes in that field when filesystem
> > uses more than one nls_table?
> 
> Consider e.g. HFS; two separate nls_table (nls_disk, nls_io), neither needs
> RCU delays of any sort.  On VFAT, for that matter - again, two tables,
> one needs RCU delay, another doesn't (both get dropped from the same
> helper, so both get it).

BTW, is there any reason not to have synchronize_rcu() in delete_module(2),
just before calling ->exit()?

It's not a hot path, unless something really weird is going on, and it
would get rid of the need to delay unload_nls() calls...