[PATCH v3 00/13] locks: saner method for managing file locks

[Jeff Layton]

v3:
- break out a ceph locking cleanup patch into a separate one earlier
  in the series
- switch back to using the i_lock to protect assignment of i_flctx.
  Using cmpxchg() was subject to races that I couldn't quite get a
  grip on. Eventually I may switch it back, but it probably doesn't
  provide much benefit.
- add a patch to clean up some comments that refer to i_flock
- use list_empty_careful in lockless checks rather than list_empty

v2:
- bugfix to the flc_posix list ordering that broke the split/merge code
- don't use i_lock to manage the i_flctx pointer. Do so locklessly
  via cmpxchg().
- reordering of the patches to make the set bisectable. As a result
  the new spinlock is not introduced until near the end of the set
- some cleanup of the lm_change operation was added, made possible
  by the move to standard list_heads for tracking locks
- it takes greater pains to avoid spinlocking by checking when the
  lists are empty in addition to whether the i_flctx pointer is NULL
- a patch was added to keep count of the number of locks, so we can
  avoid having to do count/alloc/populate in ceph and cifs

This is the third iteration of this patchset. The second was posted
on January 8th, under the cover letter entitled:

    [PATCH v2 00/10] locks: saner method for managing file locks

The big change here is that it once again uses the i_lock to protect the
i_flctx pointer assignment. In principle we should be able to use
cmpxchg instead, but that seems leave open a race that causes
locks_remove_file to miss recently-added locks. Given that is not a
super latency-sensitive codepath, an extra spinlock shouldn't make much
difference.

Many thanks to Sasha Levin who helped me nail a race that would leave
leftover locks on the i_flock list, and David Howells who convinced
me to just go ahead back to using the i_lock to protect that field.

There are some other minor changes, but overall it's pretty similar
to the last set. I still plan to merge this for v3.20.

------------------------[snip]-------------------------

We currently manage all file_locks via a singly-linked list. This is
problematic for a number of reasons:

- we have to protect all file locks with the same spinlock (or
  equivalent). Currently that uses the i_lock, but Christoph has voiced
  objections due to the potential for contention with other i_lock
  users. He'd like to see us move to using a different lock.

- we have to walk through irrelevant file locks in order to get to the
  ones we're interested in. For instance, POSIX locks are at the end
  of the list, so we have to skip over all of the flock locks and
  leases before we can work with them.

- the singly-linked list is primitive and difficult to work with. We
  have to keep track of the "before" pointer and it's easy to get that
  wrong.

Cleaning all of this up is complicated by the fact that no one really
wants to grow struct inode in order to do so. We have a single pointer
in the inode now and I don't think we want to use any more.

One possibility that Trond raised was to move this to an hlist, but
that doesn't do anything about the desire for a new spinlock.

This patchset takes the approach of replacing the i_flock list with a
new struct file_lock_context that is allocated when we intend to add a
new file lock to an inode. The file_lock_context is only freed when we
destroy the inode.

Within that, we have separate (and standard!) lists for each lock type,
and a dedicated spinlock for managing those lists. In principle we could
even consider adding separate locks for each, but I didn't bother with
that for now.

For now, the code is still pretty "raw" and isn't bisectable. This is
just a RFC for the basic approach. This is probably v3.19 material at
best.

Anyone have thoughts or comments on the basic approach?

Jeff Layton (13):
  locks: add new struct list_head to struct file_lock
  locks: have locks_release_file use flock_lock_file to release generic
    flock locks
  locks: add a new struct file_locking_context pointer to struct inode
  ceph: move spinlocking into ceph_encode_locks_to_buffer and
    ceph_count_locks
  locks: move flock locks to file_lock_context
  locks: convert posix locks to file_lock_context
  locks: convert lease handling to file_lock_context
  locks: remove i_flock field from struct inode
  locks: add a dedicated spinlock to protect i_flctx lists
  locks: clean up the lm_change prototype
  locks: keep a count of locks on the flctx lists
  locks: consolidate NULL i_flctx checks in locks_remove_file
  locks: update comments that refer to inode->i_flock

 fs/ceph/locks.c      |  64 +++---
 fs/ceph/mds_client.c |   4 -
 fs/cifs/file.c       |  34 +--
 fs/inode.c           |   3 +-
 fs/lockd/svcsubs.c   |  26 ++-
 fs/locks.c           | 569 +++++++++++++++++++++++++++------------------------
 fs/nfs/delegation.c  |  23 ++-
 fs/nfs/nfs4state.c   |  70 ++++---
 fs/nfs/pagelist.c    |   6 +-
 fs/nfs/write.c       |  41 +++-
 fs/nfsd/nfs4state.c  |  21 +-
 fs/read_write.c      |   2 +-
 include/linux/fs.h   |  52 +++--
 13 files changed, 510 insertions(+), 405 deletions(-)

-- 
2.1.0

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