linux-kernel - Re: Question about C language.

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Message-ID: <47E60702.30302@redhat.com>
Date:	Sun, 23 Mar 2008 03:30:10 -0400
From:	Chris Snook <csnook@...hat.com>
To:	Francis Moreau <francis.moro@...il.com>
CC:	linux-kernel@...r.kernel.org
Subject: Re: Question about C language.

Francis Moreau wrote:
> Hello,
> 
> 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.

-- Chris
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