Re: [PATCH 2/2] ipc,msg: provide barrier pairings for lockless receive

[Manfred Spraul]

On 05/30/2015 02:03 AM, Davidlohr Bueso wrote:
> We currently use a full barrier on the sender side to
> to avoid receiver tasks disappearing on us while still
> performing on the sender side wakeup. We lack however,
> the proper CPU-CPU interactions pairing on the receiver
> side which busy-waits for the message. Similarly, we do
> not need a full smp_mb, and can relax the semantics for
> the writer and reader sides of the message. This is safe
> as we are only ordering loads and stores to r_msg. And in
> both smp_wmb and smp_rmb, there are no stores after the
> calls _anyway_.
I like the idea, the pairing in ipc is not good.
Another one is still open in sem.

Perhaps we should formalize it a bit more, so that it is easy to find 
which barrier pair belongs together.
It is only an idea, but right now there are too many bugs.

> This obviously applies for pipelined_send and expunge_all,
> for EIRDM when destroying a queue.
>
> Signed-off-by: Davidlohr Bueso <dbueso@xxxxxxx>
> ---
>   ipc/msg.c | 31 ++++++++++++++++++++++---------
>   1 file changed, 22 insertions(+), 9 deletions(-)
>
> diff --git a/ipc/msg.c b/ipc/msg.c
> index 2b6fdbb..ac5116e 100644
> --- a/ipc/msg.c
> +++ b/ipc/msg.c
> @@ -196,7 +196,7 @@ static void expunge_all(struct msg_queue *msq, int res)
>   		 * or dealing with -EAGAIN cases. See lockless receive part 1
>   		 * and 2 in do_msgrcv().
>   		 */
> -		smp_mb();
> +		smp_wmb();
Idea for improvement: Add here /* smp_barrier_pair: ipc_msg_01 */

>   		msr->r_msg = ERR_PTR(res);
>   	}
>   }
> @@ -580,7 +580,7 @@ static inline int pipelined_send(struct msg_queue *msq, struct msg_msg *msg)
>   				/* initialize pipelined send ordering */
>   				msr->r_msg = NULL;
>   				wake_up_process(msr->r_tsk);
> -				smp_mb(); /* see barrier comment below */
> +				smp_wmb(); /* see barrier comment below */
>   				msr->r_msg = ERR_PTR(-E2BIG);
>   			} else {
>   				msr->r_msg = NULL;
> @@ -589,11 +589,12 @@ static inline int pipelined_send(struct msg_queue *msq, struct msg_msg *msg)
>   				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().
> +				 * setting r_msg, as the receiving can otherwise
> +				 * exit - once r_msg is set, the receiver can
> +				 * continue. See lockless receive part 1 and 2
> +				 * in do_msgrcv().
>   				 */
> -				smp_mb();
> +				smp_wmb();
Idea for improvement: Add here /* smp_barrier_pair: ipc_msg_02 */
>   				msr->r_msg = msg;
>   
>   				return 1;
> @@ -934,10 +935,22 @@ long do_msgrcv(int msqid, void __user *buf, size_t bufsz, long msgtyp, int msgfl
>   		 * wake_up_process(). There is a race with exit(), see
>   		 * ipc/mqueue.c for the details.
>   		 */
> -		msg = (struct msg_msg *)msr_d.r_msg;
> -		while (msg == NULL) {
> -			cpu_relax();
> +		for (;;) {
> +			/*
> +			 * Pairs with writer barrier in pipelined_send
> +			 * or expunge_all
> +			 */
> +			smp_rmb();
And here again /* smp_barrier_pair: for ipc_msg_01 and ipc_msg_02 */

--
    Manfred

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