ipc: update locking scheme comments

The initial documentation was a bit incomplete, update accordingly.

[akpm@linux-foundation.org: make it more readable in 80 columns]
Signed-off-by: Davidlohr Bueso <davidlohr@hp.com>
Acked-by: Manfred Spraul <manfred@colorfullife.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 21 insertions, 6 deletions

diff --git a/ipc/util.c b/ipc/util.c
index fdb8ae740775..7684f41bce76 100644
--- a/ipc/util.c
+++ b/ipc/util.c
@@ -17,12 +17,27 @@
  *            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)
+ *      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>