On Fri, 2014-01-10 at 10:48 -0800, Davidlohr Bueso wrote:
> 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 counter part at the receiving end is well documented for the lockless
> recv algorithm, we still need to document these specific smp_mb() calls.
>
> Signed-off-by: Davidlohr Bueso <davidl...@hp.com>
> ---
> ipc/msg.c | 19 +++++++++++++++++--
> 1 file changed, 17 insertions(+), 2 deletions(-)
>
> diff --git a/ipc/msg.c b/ipc/msg.c
> index 4377f4a..8c1cc76 100644
> --- 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 reveice part 1
^^^^^^^
lysdexic fingers
> + * 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 reveice part 1 and 2 in
^^^^^^^ ditto
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