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Date:   Mon, 6 Dec 2021 17:28:35 -0500
From:   Vivek Goyal <vgoyal@redhat.com>
To:     Eric Wong <normalperson@yhbt.net>
Cc:     fuse-devel@lists.sourceforge.net, linux-fsdevel@vger.kernel.org
Subject: Re: per-inode locks in FUSE (kernel vs userspace)
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References: <20211203000534.M766663@dcvr>
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On Fri, Dec 03, 2021 at 12:05:34AM +0000, Eric Wong wrote:
> Hi all, I'm working on a new multi-threaded FS using the
> libfuse3 fuse_lowlevel.h API.  It looks to me like the kernel
> already performs the necessary locking on a per-inode basis to
> save me some work in userspace.
> 
> In particular, I originally thought I'd need pthreads mutexes on
> a per-inode (fuse_ino_t) basis to protect userspace data
> structures between the .setattr (truncate), .fsync, and
> .write_buf userspace callbacks.
> 
> However upon reading the kernel, I can see fuse_fsync,
> fuse_{cache,direct}_write_iter in fs/fuse/file.c all use
> inode_lock.  do_truncate also uses inode_lock in fs/open.c.
> 
> So it's look like implementing extra locking in userspace would
> do nothing useful in my case, right?

I guess it probably is a good idea to implement proper locking
in multi-threaded fs and not rely on what kind of locking
kernel is doing. If kernel locking changes down the line, your
implementation will be broken.

Vivek