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        $Id: README.Locking,v 1.12 2005/04/13 13:22:35 dwmw2 Exp $

        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 semaphore, 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 Semaphore f->sem

        ---------------------

This is the JFFS2-internal equivalent of the inode semaphore 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

semaphore, 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 semaphores 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 semaphore 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:

        c->erase_completion_lock and c->inocache_lock has special ordering:

        1. c->erase_completion_lock (must be locked first)

        2. c->inocache_lock

        erase_free_sem

        --------------

This semaphore is 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 can not 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.

The erase_free_sem mutex is also used in the jffs2_mark_node_obsolete()

function which manipulates obsolete nodes (which may be removed

from the list and freed any time) and may sleep (since it reads flash).

        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.