- $Id: README.Locking,v 1.12 2005/04/13 13:22:35 dwmw2 Exp $
JFFS2 LOCKING DOCUMENTATION
---------------------------
alloc_sem
---------
-The alloc_sem is a per-filesystem semaphore, used primarily to ensure
+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
Ordering constraints: See f->sem.
- File Semaphore f->sem
+ File Mutex f->sem
---------------------
-This is the JFFS2-internal equivalent of the inode semaphore i->i_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.
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
+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 semaphores in one thread.
+ 2. Never attempt to lock two file mutexes in one thread.
No ordering rules have been made for doing so.
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
+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
erase_free_sem
--------------
-This semaphore 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
+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
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.
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