linux-kernel - Re: Question about C language.

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Date:	Mon, 24 Mar 2008 09:24:06 +0100
From:	"Francis Moreau" <francis.moro@...il.com>
To:	"Chris Snook" <csnook@...hat.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Question about C language.

Hello,

On Sun, Mar 23, 2008 at 8:30 AM, Chris Snook <csnook@...hat.com> wrote:
>  > I know it's a bit out of topic but this is something I need to clarify for
>  > writing a Linux driver... hope you don't mind.
>  >
>  > In my driver I have a global variable that controls a loop such as:
>  >
>  > int my_condition;
>  >
>  > void change_my_condition(int new)
>  > {
>  >     my_condition = new;
>  > }
>  >
>  > int foo(void)
>  > {
>  >     /* irqs are disabled */
>  >     my_condition = 1;
>  >     do {
>  >         ....
>  >         local_irq_enable();
>  >         cpu_sleep();
>  >         local_irq_disable();
>  >
>  >    } while (my_condition);
>  >
>  > }
>  >
>  > This variable is modified by an interrupt handler define in another file
>  > by using 'change_my_condition' function.
>  >
>  > By reading the ISO C99 specification, I _think_ that I needn't any
>  > kind of barrier
>  > or even use the volatile type qualifier for my_condition variable to make a true
>  > access to 'my_condition' in the controlling expression of the while, but I'm not
>  > sure.
>  >
>  > Coud anybody confirm ?
>  >
>  > Thanks,
>
>  Even volatile may be insufficient with some architecture/compiler
>  combinations.  You should use explicit barriers wherever you need them,
>  or Bad Things will happen.
>

Really ?

Could you give me some details please ?

As I said previously, I don't think I need any barriers here because this code
doesn't have any reordering issues and futhermore only one CPU will be up
at this time.

Actually the only issue I can see is about compiler aliases. I don't know if
in that case it could use an alias for 'my_condition'. Looking at the code, it
would be a bad idea but I can't find anything about this in the C specification.
The C spec talks about sequence points but I'm really confused when reading
that part...

Thanks
-- 
Francis
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