[PATCH v5 0/4] assorted ->i_count changes + extension of lockless handling

[Mateusz Guzik <mjguzik@gmail.com>]

The stock kernel support partial lockless in handling in that iput() can
decrement any value > 1. Any ref acquire however requires the spinlock.

With this patchset ref acquires when the value was already at least 1
also become lockless. That is, only transitions 0->1 and 1->0 take the
lock.

I verified when nfs calls into the hash taking the lock is typically
avoided. Similarly, btrfs likes to igrab() and avoids the lock.
However, I have to fully admit I did not perform any benchmarks. While
cleaning stuff up I noticed lockless operation is almost readily
available so I went for it.

Clean-up wise, the icount_read_once() stuff lines up with inode_state_read_once().
The prefix is different but I opted to not change it due to igrab(), ihold() et al.

There is a future-proofing change in iput_final(). I am not going to
strongly insist on it, but at the very least the problem it sorts out
needs to be noted in a comment.

v5:
- reword some commentary
- add unlikely to the new icount check in iput_final()

v4:
- squash icount_read patches
- use icount_read_once in the new ihold assert, reported by syzbot
- squash lockless ref acquire patches, rewrite new comments

v3:
- tidy up ihold
- add lockless handling to the hash

Mateusz Guzik (4):
  fs: add icount_read_once() and stop open-coding ->i_count loads
  fs: relocate and tidy up ihold()
  fs: handle potential filesystems which use I_DONTCACHE and drop the
    lock in ->drop_inode
  fs: allow lockless ->i_count bumps as long as it does not transition
    0->1

 arch/powerpc/platforms/cell/spufs/file.c |   2 +-
 fs/btrfs/inode.c                         |   2 +-
 fs/ceph/mds_client.c                     |   2 +-
 fs/dcache.c                              |   4 +
 fs/ext4/ialloc.c                         |   4 +-
 fs/hpfs/inode.c                          |   2 +-
 fs/inode.c                               | 122 ++++++++++++++++++-----
 fs/nfs/inode.c                           |   4 +-
 fs/smb/client/inode.c                    |   2 +-
 fs/ubifs/super.c                         |   2 +-
 fs/xfs/xfs_inode.c                       |   2 +-
 fs/xfs/xfs_trace.h                       |   2 +-
 include/linux/fs.h                       |  13 +++
 include/trace/events/filelock.h          |   2 +-
 security/landlock/fs.c                   |   2 +-
 15 files changed, 130 insertions(+), 37 deletions(-)

-- 
2.48.1