[JFFS2][XATTR] Add a description about c->xattr_sem

[Linux-MTD Mailing List]

Commit:     8b0b339d46ca0105a9936e3caa3bac80b72de7a3
Parent:     de1f72fab35d2b6215017690c6dc27b8f4aa14bc
Author:     KaiGai Kohei <kaigai at ak.jp.nec.com>
AuthorDate: Sat May 13 15:14:14 2006 +0900
Commit:     KaiGai Kohei <kaigai at ak.jp.nec.com>
CommitDate: Sat May 13 15:14:14 2006 +0900

    [JFFS2][XATTR] Add a description about c->xattr_sem
    
    Add a description about the c->xattr_sem read/write semaphore
    into README.Locking.
    
    [3/10] jffs2-xattr-v5.1-03-append_README.Locking.patch
    
    Signed-off-by: KaiGai Kohei <kaigai at ak.jp.nec.com>

 fs/jffs2/README.Locking |   21 +++++++++++++++++++++
 1 files changed, 21 insertions(+), 0 deletions(-)

diff --git a/fs/jffs2/README.Locking b/fs/jffs2/README.Locking
index b794343..c8f0bd6 100644
--- a/fs/jffs2/README.Locking
+++ b/fs/jffs2/README.Locking
@@ -150,3 +150,24 @@ the buffer.
 
 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.