ipc/sem.c: store which operation blocks in perform_atomic_semop()

Preparation for the next patch:

In the slow-path of perform_atomic_semop(), store a pointer to the
operation that caused the operation to block.
Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
Cc: Davidlohr Bueso <davidlohr.bueso@hp.com>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

ipc/sem.c

@@ -109,6 +109,7 @@ struct sem_queue {
 	int			pid;	 /* process id of requesting process */
 	int			status;	 /* completion status of operation */
 	struct sembuf		*sops;	 /* array of pending operations */
+	struct sembuf		*blocking; /* the operation that blocked */
 	int			nsops;	 /* number of operations */
 	int			alter;	 /* does *sops alter the array? */
 };
@@ -642,6 +643,8 @@ static int perform_atomic_semop(struct sem_array *sma, struct sem_queue *q)
 	goto undo;
 
 would_block:
+	q->blocking = sop;
+
 	if (sop->sem_flg & IPC_NOWAIT)
 		result = -EAGAIN;
 	else