linux-kernel - Re: [PATCH v3 1/2] vfs: add rcu-based find

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Date: Tue, 11 Jun 2024 15:04:52 +0200
From: Christian Brauner <brauner@...nel.org>
To: Mateusz Guzik <mjguzik@...il.com>
Cc: Jan Kara <jack@...e.cz>, viro@...iv.linux.org.uk, 
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org, linux-btrfs@...r.kernel.org, 
	josef@...icpanda.com, hch@...radead.org
Subject: Re: [PATCH v3 1/2] vfs: add rcu-based find_inode variants for iget
 ops

On Tue, Jun 11, 2024 at 01:40:37PM +0200, Mateusz Guzik wrote:
> On Tue, Jun 11, 2024 at 12:50:11PM +0200, Jan Kara wrote:
> > On Tue 11-06-24 12:16:31, Mateusz Guzik wrote:
> > > +/**
> > > + * ilookup5 - search for an inode in the inode cache
> >       ^^^ ilookup5_rcu
> > 
> 
> fixed in my branch
> 
> > > + * @sb:		super block of file system to search
> > > + * @hashval:	hash value (usually inode number) to search for
> > > + * @test:	callback used for comparisons between inodes
> > > + * @data:	opaque data pointer to pass to @test
> > > + *
> > > + * This is equivalent to ilookup5, except the @test callback must
> > > + * tolerate the inode not being stable, including being mid-teardown.
> > > + */
> > ...
> > > +struct inode *ilookup5_nowait_rcu(struct super_block *sb, unsigned long hashval,
> > > +		int (*test)(struct inode *, void *), void *data);
> > 
> > I'd prefer wrapping the above so that it fits into 80 columns.
> > 
> 
> the last comma is precisely at 80, but i can wrap it if you insist
> 
> > Otherwise feel free to add:
> > 
> > Reviewed-by: Jan Kara <jack@...e.cz>
> > 
> 
> thanks
> 
> I'm going to wait for more feedback, tweak the commit message to stress
> that this goes from 2 hash lock acquires to 1, maybe fix some typos and
> submit a v4.
> 
> past that if people want something faster they are welcome to implement
> or carry it over the finish line themselves.

I'm generally fine with this but I would think that we shouldn't add all
these helpers without any users. I'm not trying to make this a chicken
and egg problem though. Let's get the blessing from Josef to convert
btrfs to that *_rcu variant and then we can add that helper. Additional
helpers can follow as needed? @Jan, thoughts?