ipc: document general ipc locking scheme

commit 05603c44a7627793219b0bd9a7b236099dc9cd9d upstream.

As suggested by Andrew, add a generic initial locking scheme used
throughout all sysv ipc mechanisms.  Documenting the ids rwsem, how rcu
can be enough to do the initial checks and when to actually acquire the
kern_ipc_perm.lock spinlock.

I found that adding it to util.c was generic enough.

Signed-off-by: Davidlohr Bueso <davidlohr.bueso@hp.com>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Manfred Spraul <manfred@colorfullife.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/ipc/util.c b/ipc/util.c
index 5c7bd68876a8b1a133339e01bd9bda1f55600cc0..96c3fb1a6fea91e973cc18e40fe413087b5f4b4e 100644 (file)
--- a/ipc/util.c
+++ b/ipc/util.c
@@ -15,6 +15,14 @@
  * Jun 2006 - namespaces ssupport
  *            OpenVZ, SWsoft Inc.
  *            Pavel Emelianov <xemul@openvz.org>
+ *
+ * General sysv ipc locking scheme:
+ *  when doing ipc id lookups, take the ids->rwsem
+ *      rcu_read_lock()
+ *          obtain the ipc object (kern_ipc_perm)
+ *          perform security, capabilities, auditing and permission checks, etc.
+ *          acquire the ipc lock (kern_ipc_perm.lock) throught ipc_lock_object()
+ *             perform data updates (ie: SET, RMID, LOCK/UNLOCK commands)
  */
 
 #include <linux/mm.h>