per-inode locks in FUSE (kernel vs userspace)

[Eric Wong <normalperson@yhbt.net>]

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?

Thanks.