linux-kernel - Re: Why exported const value modified by another driver not updated in original driver

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Message-ID: <CAJKgH8BYe7oS5opyiX0NL7Qj3CreEiRBD4xWoORhLAVWaqMe-A@mail.gmail.com>
Date:	Tue, 4 Sep 2012 16:29:27 +0530
From:	Manavendra Nath Manav <mnm.kernel@...il.com>
To:	Julian Andres Klode <jak@...-linux.org>
Cc:	kernelnewbies@...nelnewbies.org, devel@...verdev.osuosl.org,
	Greg KH <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org, alan@...rguk.ukuu.org.uk
Subject: Re: Why exported const value modified by another driver not updated
 in original driver

On Tue, Sep 4, 2012 at 4:09 PM, Julian Andres Klode <jak@...-linux.org> wrote:
> On Tue, Sep 04, 2012 at 03:58:20PM +0530, Manavendra Nath Manav wrote:
>> On Tue, Sep 4, 2012 at 3:00 PM, Manavendra Nath Manav
>> <mnm.kernel@...il.com> wrote:
>> > Hi,
>> >
>> > I have declared a static const int variable in one driver and exported
>> > that variable symbol. In another driver i am modifying that variable.
>
> No, you did not export it. It's static (and static is kind of the opposite
> of extern). And then you try to assign a value to a non-static variable with
> the same name, which might not even exist (as the other is static, this should
> not even work in my opinion).
>
>> > The other driver prints the modified value but the original driver
>> > retains the original value. When both virtual and physical addresses
>> > of the variable as seen by both drivers are same, how is this even
>> > possible. Is it a kernel bug?
>
> It only works because the compiler optimized the value = 123 away,
> and replaced value with 123 in the first module everywhere, as it
> is declared const and thus cannot be changed, so there's no reason
> to read it from memory.
>
> --
> Julian Andres Klode  - Debian Developer, Ubuntu Member
>
> See http://wiki.debian.org/JulianAndresKlode and http://jak-linux.org/.

Still I am not able to understand !! Why the output behaviour changes
when the declaration is made as "const volatile". I get the same
results as above when i remove "static" and retain "const".
-- 
Manavendra Nath Manav
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