linux-kernel - Re: Question about C language.

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Message-ID: <47E60773.2040409@redhat.com>
Date:	Sun, 23 Mar 2008 03:32:03 -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.

Chris Snook wrote:
> 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
> 

Oops, forgot to mention, you should use atomic_t, to avoid aliasing 
problems, and ALSO use explicit barriers wherever you need them.

-- Chris
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