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			Lock-ahead translator
                        ---------------------

The objective of the lock-ahead translator is to speculatively
hold locks (inodelk and entrylk) on the universal set (0 - infinity
in case of inodelk and all basenames in case of entrylk) even
when a lock is requested only on a subset, in anticipation that
further locks will be requested within the same universal set.

So, for example, when cluster/replicate locks a region before
writing to it, lock-ahead would instead lock the entire file.
On further writes, lock-ahead can immediately return success for 
the lock requests, since the entire file has been previously locked.

To avoid starvation of other clients/mountpoints, we employ a
notify mechanism, described below.

typedef struct {
        struct list_head subset_locks;
} la_universal_lock_t;

Universal lock structure is stored in the inode context.

typedef struct {
	enum {LOCK_AHEAD_ENTRYLK, LOCK_AHEAD_FENTRYLK,
              LOCK_AHEAD_INODELK, LOCK_AHEAD_FINODELK};

        union {
	      fd_t *fd;
              loc_t loc;
	};

	off_t l_start;
        off_t l_len;

	const char *basename;

	struct list_head universal_lock;
} la_subset_lock_t;


fops implemented:

* inodelk/finodelk/entrylk/fentrylk:

lock:
  if universal lock held:
    add subset to it (save loc_t or fd) and return success
  else:
    send lock-notify fop
    hold universal lock and return
      (set inode context, add subset to it, save loc_t or fd)
    
    if this fails:
      forward the lock request

unlock:
  if subset exists in universal lock:
    delete subset lock from list
  else:
    forward it

* release:
    hold subset locks (each subset lock using the saved loc_t or fd)
    and release universal lock

* lock-notify (on unwind) (new fop)
    hold subset locks and release universal lock


lock-notify in locks translator:

if a subset lock in entrylk/inodelk cannot be satisfied
because of a universal lock held by someone else:
    unwind the lock-notify fop

==============================================
$ Last updated: Tue Feb 17 11:31:18 IST 2009 $
$ Author: Vikas Gorur <vikas@zresearch.com>  $
==============================================