BOUNCE rtl@rtlinux.org: Approval required: Non-member submissionfrom ["chandu Reddy" <chandureddy@hclt.com>] (fwd)

[Der Herr Hofrat <der.herr@hofr.at>]

>From owner-rtl Fri May 18 01:36:57 2001
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From: "chandu Reddy" <chandureddy@hclt.com>
To: <rtl@fsmlabs.com>
References: <20010515215032.CB08610AA0@denx.denx.de> <3B024373.99329173@robo.fe.uni-lj.si>
Subject: Re: [rtl] Shared memory synchronization
Date: Fri, 18 May 2001 13:03:34 +0100
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HI
  regarding Shared memory synchronisation,
  u can refer the article in Dr. Dobb's Journal  dated November 1999.
  it is given  with out using mbuff driver. u can use the same concept
here.


Chandu
These are my personal interests and openions.


----- Original Message -----
From: Ales Bardorfer <alesb@robo.fe.uni-lj.si>
To: <rtl@fsmlabs.com>
Sent: Wednesday, May 16, 2001 10:08 AM
Subject: [rtl] Shared memory synchronization


> I posted this before, but it seems there have been some problems with
> the mailing list. So, here it is again:
>
> I have some problems synchronizing shared mem access.
>
> My RT-thread uses shared memory (mbuff) to get some info what to do. I
> would like to be changing this shared memory from user space. Thus to
> ensure the atomic changes of the shm data, I created two identical
> structures in shm and a variable to point to the valid part of the data.
> >From user space I allways change the "other part", the one which is not
> being used by RT-thread. Like this:
>
> struct {
>         int valid_part;
>         struct actual_data[2];
> } my_data;
>
> shm = (my_data *)mbuff_alloc(...)
>
> Periodic RT-thread:
>         pthread_wait_np();
>         int valid_part = shm->valid_part;
>         /* Read from and write to shm */
>         a = shm->actual_data[valid_part].something;
>         shm->actual_data[valid_part].something_else = b;
>         ...
>
> User space:
>         shm = (my_data *)mbuff_alloc(...)
>         /* We are only allowed to change the non valid part! */
>         int non_valid_part = !shm->valid_part;
>
>         /* Change data in shm */
>         shm->actual_data[non_valid_part].something = changed_value;
>         ...
>
>         /* At the end change the pointer! */
>         shm->valid_part = non_valid_part;
>
>
> The main problem is the changing of this pointer (shm->valid_part). I
> guess this should not be the problem on a 1-CPU machine, but on a SMP I
> get lots of errors.
> What synchonizing mechanism should I use? Are mutexes or semaphores the
> right answer for that? I am relatively new to mutexes and semaphores,
> but is it safe to use mutexes, which might block the execution, in
> RT-thread? I would only like to (efficiently = no CPU load) block the
> user space thread, to wait for the RT-thread to finish, and then the
> user space thread can safely change the pointer.
>
> Is that possible and how? Is my approach with two identical structures
> and a "valid pointer" right, or should I use some other approach?
>
> Regards,
> Ales
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