IPC/semaphores: move the rwmutex handling inside semctl_down

semctl_down is called with the rwmutex (the one which protects the list of
ipcs) taken in write mode.

This patch moves this rwmutex taken in write-mode inside semctl_down.

This has the advantages of reducing a little bit the window during which this
rwmutex is taken, clarifying sys_semctl, and finally of having a coherent
behaviour with [shm|msg]ctl_down

Signed-off-by: Pierre Peiffer <pierre.peiffer@bull.net>
Acked-by: Serge Hallyn <serue@us.ibm.com>
Cc: Nadia Derbey <Nadia.Derbey@bull.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

ipc/sem.c

@@ -875,6 +875,11 @@ static inline unsigned long copy_semid_from_user(struct sem_setbuf *out, void __
 	}
 }
 
+/*
+ * This function handles some semctl commands which require the rw_mutex
+ * to be held in write mode.
+ * NOTE: no locks must be held, the rw_mutex is taken inside this function.
+ */
 static int semctl_down(struct ipc_namespace *ns, int semid, int semnum,
 		int cmd, int version, union semun arg)
 {
@@ -887,9 +892,12 @@ static int semctl_down(struct ipc_namespace *ns, int semid, int semnum,
 		if(copy_semid_from_user (&setbuf, arg.buf, version))
 			return -EFAULT;
 	}
+	down_write(&sem_ids(ns).rw_mutex);
 	sma = sem_lock_check_down(ns, semid);
-	if (IS_ERR(sma))
-		return PTR_ERR(sma);
+	if (IS_ERR(sma)) {
+		err = PTR_ERR(sma);
+		goto out_up;
+	}
 
 	ipcp = &sma->sem_perm;
 
@@ -915,26 +923,22 @@ static int semctl_down(struct ipc_namespace *ns, int semid, int semnum,
 	switch(cmd){
 	case IPC_RMID:
 		freeary(ns, ipcp);
-		err = 0;
-		break;
+		goto out_up;
 	case IPC_SET:
 		ipcp->uid = setbuf.uid;
 		ipcp->gid = setbuf.gid;
 		ipcp->mode = (ipcp->mode & ~S_IRWXUGO)
 				| (setbuf.mode & S_IRWXUGO);
 		sma->sem_ctime = get_seconds();
-		sem_unlock(sma);
-		err = 0;
 		break;
 	default:
-		sem_unlock(sma);
 		err = -EINVAL;
-		break;
 	}
-	return err;
 
 out_unlock:
 	sem_unlock(sma);
+out_up:
+	up_write(&sem_ids(ns).rw_mutex);
 	return err;
 }
 
@@ -968,9 +972,7 @@ asmlinkage long sys_semctl (int semid, int semnum, int cmd, union semun arg)
 		return err;
 	case IPC_RMID:
 	case IPC_SET:
-		down_write(&sem_ids(ns).rw_mutex);
 		err = semctl_down(ns,semid,semnum,cmd,version,arg);
-		up_write(&sem_ids(ns).rw_mutex);
 		return err;
 	default:
 		return -EINVAL;