Re: [PATCH 4/5] ipc,msg: document volatile r_msg

[Manfred Spraul <manfred@colorfullife.com>]

Hi Davidlohr,

On 05/13/2014 10:27 PM, Davidlohr Bueso wrote:
> The need for volatile is not obvious, document it.
>
> Signed-off-by: Davidlohr Bueso <davidlohr@hp.com>
Signed-off-by: Manfred Spraul <manfred@colorfullife.com>

In the long run, it would be great if the logic from sem.c would be 
moved to one central place.
- wake_up_sem_queue_prepare()
- wake_up_sem_queue_do()
- get_queue_result()
- the realtime compatible implementation that uses a completion instead 
of the busy loop in get_queue_result()

The code more or less just a reliable wait queue, nothing is specific to 
sysv:
- the woken up task knows if it was woken up due to a signal or due to 
wake_up_process()
- wake_up_process() out of line, after dropping all locks
- only two lock operation (one for: check that task must wait and go to 
sleep, one for: identify the task that must be woken up).

With a "normal" wait queue, there are three lock operation (notice that 
task was woken up, cleanup)


> ---
>   ipc/msg.c | 10 +++++++---
>   1 file changed, 7 insertions(+), 3 deletions(-)
>
> diff --git a/ipc/msg.c b/ipc/msg.c
> index c2cdb5b..956cd65 100644
> --- a/ipc/msg.c
> +++ b/ipc/msg.c
> @@ -42,9 +42,7 @@
>   #include <linux/uaccess.h>
>   #include "util.h"
>   
> -/*
> - * one msg_receiver structure for each sleeping receiver:
> - */
> +/* one msg_receiver structure for each sleeping receiver */
>   struct msg_receiver {
>   	struct list_head	r_list;
>   	struct task_struct	*r_tsk;
> @@ -54,6 +52,12 @@ struct msg_receiver {
>   	long			r_msgtype;
>   	long			r_maxsize;
>   
> +	/*
> +	 * Mark r_msg volatile so that the compiler
> +	 * does not try to get smart and optimize
> +	 * it. We rely on this for the lockless
> +	 * receive algorithm.
> +	 */
>   	struct msg_msg		*volatile r_msg;
>   };
>