vserver 1.9.5.x5

[linux-2.6.git] / fs / jffs2 / README.Locking

diff --git a/fs/jffs2/README.Locking b/fs/jffs2/README.Locking
index 72dbcea..49771cf 100644 (file)
--- a/fs/jffs2/README.Locking
+++ b/fs/jffs2/README.Locking
@@ -1,4 +1,4 @@
-       $Id: README.Locking,v 1.4 2002/03/08 16:20:06 dwmw2 Exp $
+       $Id: README.Locking,v 1.9 2004/11/20 10:35:40 dwmw2 Exp $

 
        JFFS2 LOCKING DOCUMENTATION
        ---------------------------
 
        JFFS2 LOCKING DOCUMENTATION
        ---------------------------


@@ -80,10 +80,10 @@ 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.
 
 (NB) the per-inode list of physical nodes. The latter is a special
 case - see below.
 

-As the MTD API permits erase-completion callback functions to be
-called from bottom-half (timer) context, and these functions access
-the data structures protected by this lock, it must be locked with
-spin_lock_bh().
+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
 
 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


@@ -99,8 +99,27 @@ 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.
 
 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.
 

-       node_free_sem
-       -------------
+
+       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.
+
+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 semaphore is only used by the erase code which frees obsolete
 node references and the jffs2_garbage_collect_deletion_dirent()
 
 This semaphore is only used by the erase code which frees obsolete
 node references and the jffs2_garbage_collect_deletion_dirent()


@@ -114,3 +133,16 @@ 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.
 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.