- $Id: README.Locking,v 1.9 2004/11/20 10:35:40 dwmw2 Exp $
+ $Id: README.Locking,v 1.12 2005/04/13 13:22:35 dwmw2 Exp $
JFFS2 LOCKING DOCUMENTATION
---------------------------
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:
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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