[PATCH v3 0/7] 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 needs to be
noted in a comment.

v2:
- tidy up ihold
- add lockless handling to the hash

Mateusz Guzik (7):
  fs: add icount_read_once()
  Use icount_read() and icount_read_once() as appropriate.
  fs: enforce locking in icount_read(), add some commentary
  fs: relocate and tidy up ihold()
  fs: handle hypothetical filesystems which use I_DONTCACHE and drop the
    lock in ->drop_inode
  fs: locklessly bump refs in igrab as long as it does not transition
    0->1
  fs: locklessly bump refs in the inode hash when possible

 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                               | 114 ++++++++++++++++++-----
 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, 122 insertions(+), 37 deletions(-)

-- 
2.48.1