ipc,sem: open code and rename sem_lock

Rename sem_lock() to sem_obtain_lock(), so we can introduce a sem_lock()
later that only locks the sem_array and does nothing else.

Open code the locking from ipc_lock() in sem_obtain_lock() so we can
introduce finer grained locking for the sem_array in the next patch.

[akpm@linux-foundation.org: propagate the ipc_obtain_object() errno out of sem_obtain_lock()]
Signed-off-by: Rik van Riel <riel@redhat.com>
Acked-by: Davidlohr Bueso <davidlohr.bueso@hp.com>
Cc: Chegu Vinod <chegu_vinod@hp.com>
Cc: Emmanuel Benisty <benisty.e@gmail.com>
Cc: Jason Low <jason.low2@hp.com>
Cc: Michel Lespinasse <walken@google.com>
Cc: Peter Hurley <peter@hurleysoftware.com>
Cc: Stanislav Kinsbursky <skinsbursky@parallels.com>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

ipc/sem.c

@@ -194,14 +194,31 @@ void __init sem_init (void)
  * sem_lock_(check_) routines are called in the paths where the rw_mutex
  * is not held.
  */
-static inline struct sem_array *sem_lock(struct ipc_namespace *ns, int id)
+static inline struct sem_array *sem_obtain_lock(struct ipc_namespace *ns, int id)
 {
-	struct kern_ipc_perm *ipcp = ipc_lock(&sem_ids(ns), id);
+	struct kern_ipc_perm *ipcp;
+	struct sem_array *sma;
 
-	if (IS_ERR(ipcp))
-		return (struct sem_array *)ipcp;
+	rcu_read_lock();
+	ipcp = ipc_obtain_object(&sem_ids(ns), id);
+	if (IS_ERR(ipcp)) {
+		sma = ERR_CAST(ipcp);
+		goto err;
+	}
 
-	return container_of(ipcp, struct sem_array, sem_perm);
+	spin_lock(&ipcp->lock);
+
+	/* ipc_rmid() may have already freed the ID while sem_lock
+	 * was spinning: verify that the structure is still valid
+	 */
+	if (!ipcp->deleted)
+		return container_of(ipcp, struct sem_array, sem_perm);
+
+	spin_unlock(&ipcp->lock);
+	sma = ERR_PTR(-EINVAL);
+err:
+	rcu_read_unlock();
+	return sma;
 }
 
 static inline struct sem_array *sem_obtain_object(struct ipc_namespace *ns, int id)
@@ -1593,7 +1610,7 @@ SYSCALL_DEFINE4(semtimedop, int, semid, struct sembuf __user *, tsops,
 		goto out_free;
 	}
 
-	sma = sem_lock(ns, semid);
+	sma = sem_obtain_lock(ns, semid);
 
 	/*
 	 * Wait until it's guaranteed that no wakeup_sem_queue_do() is ongoing.