JFFS2 LOCKING DOCUMENTATION
  	---------------------------
  
  At least theoretically, JFFS2 does not require the Big Kernel Lock
  (BKL), which was always helpfully obtained for it by Linux 2.4 VFS
  code. It has its own locking, as described below.
  
  This document attempts to describe the existing locking rules for
  JFFS2. It is not expected to remain perfectly up to date, but ought to
  be fairly close.
  
  
  	alloc_sem
  	---------
  
  The alloc_sem is a per-filesystem mutex, used primarily to ensure
  contiguous allocation of space on the medium. It is automatically
  obtained during space allocations (jffs2_reserve_space()) and freed
  upon write completion (jffs2_complete_reservation()). Note that
  the garbage collector will obtain this right at the beginning of
  jffs2_garbage_collect_pass() and release it at the end, thereby
  preventing any other write activity on the file system during a
  garbage collect pass.
  
  When writing new nodes, the alloc_sem must be held until the new nodes
  have been properly linked into the data structures for the inode to
  which they belong. This is for the benefit of NAND flash - adding new
  nodes to an inode may obsolete old ones, and by holding the alloc_sem
  until this happens we ensure that any data in the write-buffer at the
  time this happens are part of the new node, not just something that
  was written afterwards. Hence, we can ensure the newly-obsoleted nodes
  don't actually get erased until the write-buffer has been flushed to
  the medium.
  
  With the introduction of NAND flash support and the write-buffer, 
  the alloc_sem is also used to protect the wbuf-related members of the
  jffs2_sb_info structure. Atomically reading the wbuf_len member to see
  if the wbuf is currently holding any data is permitted, though.
  
  Ordering constraints: See f->sem.
  
  
  	File Mutex f->sem
  	---------------------
  
  This is the JFFS2-internal equivalent of the inode mutex i->i_sem.
  It protects the contents of the jffs2_inode_info private inode data,
  including the linked list of node fragments (but see the notes below on
  erase_completion_lock), etc.
  
  The reason that the i_sem itself isn't used for this purpose is to
  avoid deadlocks with garbage collection -- the VFS will lock the i_sem
  before calling a function which may need to allocate space. The
  allocation may trigger garbage-collection, which may need to move a
  node belonging to the inode which was locked in the first place by the
  VFS. If the garbage collection code were to attempt to lock the i_sem
  of the inode from which it's garbage-collecting a physical node, this
  lead to deadlock, unless we played games with unlocking the i_sem
  before calling the space allocation functions.
  
  Instead of playing such games, we just have an extra internal
  mutex, which is obtained by the garbage collection code and also
  by the normal file system code _after_ allocation of space.
  
  Ordering constraints: 
  
  	1. Never attempt to allocate space or lock alloc_sem with 
  	   any f->sem held.
  	2. Never attempt to lock two file mutexes in one thread.
  	   No ordering rules have been made for doing so.
  
  
  	erase_completion_lock spinlock
  	------------------------------
  
  This is used to serialise access to the eraseblock lists, to the
  per-eraseblock lists of physical jffs2_raw_node_ref structures, and
  (NB) the per-inode list of physical nodes. The latter is a special
  case - see below.
  
  As the MTD API no longer permits erase-completion callback functions
  to be called from bottom-half (timer) context (on the basis that nobody
  ever actually implemented such a thing), it's now sufficient to use
  a simple spin_lock() rather than spin_lock_bh().
  
  Note that the per-inode list of physical nodes (f->nodes) is a special
  case. Any changes to _valid_ nodes (i.e. ->flash_offset & 1 == 0) in
  the list are protected by the file mutex f->sem. But the erase code
  may remove _obsolete_ nodes from the list while holding only the
  erase_completion_lock. So you can walk the list only while holding the
  erase_completion_lock, and can drop the lock temporarily mid-walk as
  long as the pointer you're holding is to a _valid_ node, not an
  obsolete one.
  
  The erase_completion_lock is also used to protect the c->gc_task
  pointer when the garbage collection thread exits. The code to kill the
  GC thread locks it, sends the signal, then unlocks it - while the GC
  thread itself locks it, zeroes c->gc_task, then unlocks on the exit path.
  
  
  	inocache_lock spinlock
  	----------------------
  
  This spinlock protects the hashed list (c->inocache_list) of the
  in-core jffs2_inode_cache objects (each inode in JFFS2 has the
  correspondent jffs2_inode_cache object). So, the inocache_lock
  has to be locked while walking the c->inocache_list hash buckets.
  
  This spinlock also covers allocation of new inode numbers, which is
  currently just '++->highest_ino++', but might one day get more complicated
  if we need to deal with wrapping after 4 milliard inode numbers are used.
  
  Note, the f->sem guarantees that the correspondent jffs2_inode_cache
  will not be removed. So, it is allowed to access it without locking
  the inocache_lock spinlock. 
  
  Ordering constraints: 
  
  	If both erase_completion_lock and inocache_lock are needed, the
  	c->erase_completion has to be acquired first.
  
  
  	erase_free_sem
  	--------------
  
  This mutex is only used by the erase code which frees obsolete node
  references and the jffs2_garbage_collect_deletion_dirent() function.
  The latter function on NAND flash must read _obsolete_ nodes to
  determine whether the 'deletion dirent' under consideration can be
  discarded or whether it is still required to show that an inode has
  been unlinked. Because reading from the flash may sleep, the
  erase_completion_lock cannot be held, so an alternative, more
  heavyweight lock was required to prevent the erase code from freeing
  the jffs2_raw_node_ref structures in question while the garbage
  collection code is looking at them.
  
  Suggestions for alternative solutions to this problem would be welcomed.
  
  
  	wbuf_sem
  	--------
  
  This read/write semaphore protects against concurrent access to the
  write-behind buffer ('wbuf') used for flash chips where we must write
  in blocks. It protects both the contents of the wbuf and the metadata
  which indicates which flash region (if any) is currently covered by 
  the buffer.
  
  Ordering constraints:
  	Lock wbuf_sem last, after the alloc_sem or and f->sem.
  
  
  	c->xattr_sem
  	------------
  
  This read/write semaphore protects against concurrent access to the
  xattr related objects which include stuff in superblock and ic->xref.
  In read-only path, write-semaphore is too much exclusion. It's enough
  by read-semaphore. But you must hold write-semaphore when updating,
  creating or deleting any xattr related object.
  
  Once xattr_sem released, there would be no assurance for the existence
  of those objects. Thus, a series of processes is often required to retry,
  when updating such a object is necessary under holding read semaphore.
  For example, do_jffs2_getxattr() holds read-semaphore to scan xref and
  xdatum at first. But it retries this process with holding write-semaphore
  after release read-semaphore, if it's necessary to load name/value pair
  from medium.
  
  Ordering constraints:
  	Lock xattr_sem last, after the alloc_sem.