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Date: Sun, 18 May 2014 08:08:22 +0200
From: Manfred Spraul <manfred@...orfullife.com>
To: Davidlohr Bueso <davidlohr@...com>, akpm@...ux-foundation.org
CC: aswin@...com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/5] ipc,msg: document volatile r_msg
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@...com>
Signed-off-by: Manfred Spraul <manfred@...orfullife.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;
> };
>
--
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