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: Davidlohr 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: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/ipc/msg.c b/ipc/msg.c
index 4377f4a456259fc8e8dd4a97816d5128b8932ac0..245db1140ad66a2be47744f02ef0d80deea5006f 100644 (file)
--- a/ipc/msg.c
+++ b/ipc/msg.c
@@ -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)) {