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Commit ffa571da authored by Davidlohr Bueso's avatar Davidlohr Bueso Committed by Linus Torvalds
Browse files

ipc,msg: document barriers 



Both expunge_all() and pipeline_send() rely on both a nil msg value and
a full barrier to guarantee the correct ordering when waking up a task.

While its counterpart at the receiving end is well documented for the
lockless recv algorithm, we still need to document these specific
smp_mb() calls.

[akpm@linux-foundation.org: fix typo, per Mike]
[akpm@linux-foundation.org: mroe tpyos]
Signed-off-by: default avatarDavidlohr Bueso <davidlohr@hp.com>
Cc: Aswin Chandramouleeswaran <aswin@hp.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Manfred Spraul <manfred@colorfullife.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent daf948c7

ipc/msg.c

+17 −2
Original line number Diff line number Diff line
@@ -253,8 +253,14 @@ static void expunge_all(struct msg_queue *msq, int res) 
	struct msg_receiver *msr, *t;



	list_for_each_entry_safe(msr, t, &msq->q_receivers, r_list) {

		msr->r_msg = NULL;

		msr->r_msg = NULL; /* initialize expunge ordering */

		wake_up_process(msr->r_tsk);

		/*

		 * Ensure that the wakeup is visible before setting r_msg as

		 * the receiving end depends on it: either spinning on a nil,

		 * or dealing with -EAGAIN cases. See lockless receive part 1

		 * and 2 in do_msgrcv().

		 */

		smp_mb();

		msr->r_msg = ERR_PTR(res);

	}
@@ -638,15 +644,22 @@ static inline int pipelined_send(struct msg_queue *msq, struct msg_msg *msg) 


			list_del(&msr->r_list);

			if (msr->r_maxsize < msg->m_ts) {

				/* initialize pipelined send ordering */

				msr->r_msg = NULL;

				wake_up_process(msr->r_tsk);

				smp_mb();

				smp_mb(); /* see barrier comment below */

				msr->r_msg = ERR_PTR(-E2BIG);

			} else {

				msr->r_msg = NULL;

				msq->q_lrpid = task_pid_vnr(msr->r_tsk);

				msq->q_rtime = get_seconds();

				wake_up_process(msr->r_tsk);

				/*

				 * Ensure that the wakeup is visible before

				 * setting r_msg, as the receiving end depends

				 * on it. See lockless receive part 1 and 2 in

				 * do_msgrcv().

				 */

				smp_mb();

				msr->r_msg = msg;
@@ -654,6 +667,7 @@ static inline int pipelined_send(struct msg_queue *msq, struct msg_msg *msg) 
			}

		}

	}



	return 0;

}
@@ -716,6 +730,7 @@ long do_msgsnd(int msqid, long mtype, void __user *mtext, 
			goto out_unlock0;

		}



		/* enqueue the sender and prepare to block */

		ss_add(msq, &s);



		if (!ipc_rcu_getref(msq)) {