[3.11.y.z extended stable] Patch "ipc: update locking scheme comments" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    ipc: update locking scheme comments

to the linux-3.11.y-queue branch of the 3.11.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.11.y-queue

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.11.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From 6c7153716a6c4379a006eb8d5fbb2c8abb3f7e11 Mon Sep 17 00:00:00 2001
From: Davidlohr Bueso <davidlohr at hp.com>
Date: Wed, 16 Oct 2013 13:46:45 -0700
Subject: ipc: update locking scheme comments

commit 18ccee263c7e250a57f01c9434658f11f4118a64 upstream.

The initial documentation was a bit incomplete, update accordingly.

[akpm at linux-foundation.org: make it more readable in 80 columns]
Signed-off-by: Davidlohr Bueso <davidlohr at hp.com>
Acked-by: Manfred Spraul <manfred at colorfullife.com>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 ipc/util.c | 27 +++++++++++++++++++++------
 1 file changed, 21 insertions(+), 6 deletions(-)

diff --git a/ipc/util.c b/ipc/util.c
index fdb8ae7..7684f41 100644
--- a/ipc/util.c
+++ b/ipc/util.c
@@ -17,12 +17,27 @@
  *            Pavel Emelianov <xemul at 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)
+ *	rcu_read_lock()
+ *          obtain the ipc object (kern_ipc_perm) by looking up the id in an idr
+ *	    tree.
+ *	    - perform initial checks (capabilities, auditing and permission,
+ *	      etc).
+ *	    - perform read-only operations, such as STAT, INFO commands.
+ *	      acquire the ipc lock (kern_ipc_perm.lock) through
+ *	      ipc_lock_object()
+ *		- perform data updates, such as SET, RMID commands and
+ *		  mechanism-specific operations (semop/semtimedop,
+ *		  msgsnd/msgrcv, shmat/shmdt).
+ *	    drop the ipc lock, through ipc_unlock_object().
+ *	rcu_read_unlock()
+ *
+ *  The ids->rwsem must be taken when:
+ *	- creating, removing and iterating the existing entries in ipc
+ *	  identifier sets.
+ *	- iterating through files under /proc/sysvipc/
+ *
+ *  Note that sems have a special fast path that avoids kern_ipc_perm.lock -
+ *  see sem_lock().
  */

 #include <linux/mm.h>
--
1.8.3.2