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$Id: README.Locking,v 1.11 2004/11/24 09:25:09 dedekind Exp $
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JFFS2 LOCKING DOCUMENTATION
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---------------------------
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At least theoretically, JFFS2 does not require the Big Kernel Lock
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(BKL), which was always helpfully obtained for it by Linux 2.4 VFS
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code. It has its own locking, as described below.
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This document attempts to describe the existing locking rules for
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JFFS2. It is not expected to remain perfectly up to date, but ought to
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The alloc_sem is a per-filesystem semaphore, used primarily to ensure
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contiguous allocation of space on the medium. It is automatically
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obtained during space allocations (jffs2_reserve_space()) and freed
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upon write completion (jffs2_complete_reservation()). Note that
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the garbage collector will obtain this right at the beginning of
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jffs2_garbage_collect_pass() and release it at the end, thereby
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preventing any other write activity on the file system during a
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When writing new nodes, the alloc_sem must be held until the new nodes
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have been properly linked into the data structures for the inode to
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which they belong. This is for the benefit of NAND flash - adding new
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nodes to an inode may obsolete old ones, and by holding the alloc_sem
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until this happens we ensure that any data in the write-buffer at the
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time this happens are part of the new node, not just something that
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was written afterwards. Hence, we can ensure the newly-obsoleted nodes
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don't actually get erased until the write-buffer has been flushed to
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With the introduction of NAND flash support and the write-buffer,
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the alloc_sem is also used to protect the wbuf-related members of the
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jffs2_sb_info structure. Atomically reading the wbuf_len member to see
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if the wbuf is currently holding any data is permitted, though.
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Ordering constraints: See f->sem.
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This is the JFFS2-internal equivalent of the inode semaphore i->i_sem.
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It protects the contents of the jffs2_inode_info private inode data,
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including the linked list of node fragments (but see the notes below on
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erase_completion_lock), etc.
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The reason that the i_sem itself isn't used for this purpose is to
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avoid deadlocks with garbage collection -- the VFS will lock the i_sem
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before calling a function which may need to allocate space. The
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allocation may trigger garbage-collection, which may need to move a
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node belonging to the inode which was locked in the first place by the
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VFS. If the garbage collection code were to attempt to lock the i_sem
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of the inode from which it's garbage-collecting a physical node, this
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lead to deadlock, unless we played games with unlocking the i_sem
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before calling the space allocation functions.
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Instead of playing such games, we just have an extra internal
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semaphore, which is obtained by the garbage collection code and also
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by the normal file system code _after_ allocation of space.
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1. Never attempt to allocate space or lock alloc_sem with
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2. Never attempt to lock two file semaphores in one thread.
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No ordering rules have been made for doing so.
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erase_completion_lock spinlock
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------------------------------
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This is used to serialise access to the eraseblock lists, to the
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per-eraseblock lists of physical jffs2_raw_node_ref structures, and
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(NB) the per-inode list of physical nodes. The latter is a special
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As the MTD API no longer permits erase-completion callback functions
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to be called from bottom-half (timer) context (on the basis that nobody
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ever actually implemented such a thing), it's now sufficient to use
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a simple spin_lock() rather than spin_lock_bh().
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Note that the per-inode list of physical nodes (f->nodes) is a special
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case. Any changes to _valid_ nodes (i.e. ->flash_offset & 1 == 0) in
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the list are protected by the file semaphore f->sem. But the erase
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code may remove _obsolete_ nodes from the list while holding only the
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erase_completion_lock. So you can walk the list only while holding the
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erase_completion_lock, and can drop the lock temporarily mid-walk as
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long as the pointer you're holding is to a _valid_ node, not an
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The erase_completion_lock is also used to protect the c->gc_task
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pointer when the garbage collection thread exits. The code to kill the
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GC thread locks it, sends the signal, then unlocks it - while the GC
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thread itself locks it, zeroes c->gc_task, then unlocks on the exit path.
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inocache_lock spinlock
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----------------------
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This spinlock protects the hashed list (c->inocache_list) of the
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in-core jffs2_inode_cache objects (each inode in JFFS2 has the
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correspondent jffs2_inode_cache object). So, the inocache_lock
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has to be locked while walking the c->inocache_list hash buckets.
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Note, the f->sem guarantees that the correspondent jffs2_inode_cache
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will not be removed. So, it is allowed to access it without locking
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the inocache_lock spinlock.
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Ordering constraints:
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c->erase_completion_lock and c->inocache_lock has special ordering:
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1. c->erase_completion_lock (must be locked first)
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This semaphore is used by the erase code which frees obsolete
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node references and the jffs2_garbage_collect_deletion_dirent()
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function. The latter function on NAND flash must read _obsolete_ nodes
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to determine whether the 'deletion dirent' under consideration can be
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discarded or whether it is still required to show that an inode has
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been unlinked. Because reading from the flash may sleep, the
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erase_completion_lock can not be held, so an alternative, more
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heavyweight lock was required to prevent the erase code from freeing
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the jffs2_raw_node_ref structures in question while the garbage
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collection code is looking at them.
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The erase_free_sem mutex is also used in the jffs2_mark_node_obsolete()
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function which manipulates obsolete nodes (which may be removed
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from the list and freed any time) and may sleep (since it reads flash).
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This read/write semaphore protects against concurrent access to the
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write-behind buffer ('wbuf') used for flash chips where we must write
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in blocks. It protects both the contents of the wbuf and the metadata
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which indicates which flash region (if any) is currently covered by
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Ordering constraints:
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Lock wbuf_sem last, after the alloc_sem or and f->sem.
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fs/jffs2/README.Locking
