Re: Why exported const value modified by another driver not updated in original driver
On Tue, Sep 4, 2012 at 5:55 PM, Dan Carpenter wrote: > On Tue, Sep 04, 2012 at 03:58:20PM +0530, Manavendra Nath Manav wrote: >> Is the above a genuine kernel bug, or i am missing something out here. Pls >> help. >> > > When you declare something as const then the compiler assumes it > really is const and uses a literal instead of reading from memory. > I'm surprised the compiler doesn't print a warning message. > > It has to do with compilers, nothing to do with kernels. > > regards, > dan carpenter Thanks All, I understood the problem and current gcc behaviour after looking at output of objdump of driver.ko file when the variable is declared as "const" and in second case as "const volatile". The compiler optimises by directly passing the value in first case and the address of variable in second case. Thanks for all the help and clarification. push $0x7b // 123 in decimal push $0x0 -- Manavendra Nath Manav -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: Why exported const value modified by another driver not updated in original driver
On Tue, Sep 4, 2012 at 5:55 PM, Dan Carpenter dan.carpen...@oracle.com wrote: On Tue, Sep 04, 2012 at 03:58:20PM +0530, Manavendra Nath Manav wrote: Is the above a genuine kernel bug, or i am missing something out here. Pls help. When you declare something as const then the compiler assumes it really is const and uses a literal instead of reading from memory. I'm surprised the compiler doesn't print a warning message. It has to do with compilers, nothing to do with kernels. regards, dan carpenter Thanks All, I understood the problem and current gcc behaviour after looking at output of objdump of driver.ko file when the variable is declared as const and in second case as const volatile. The compiler optimises by directly passing the value in first case and the address of variable in second case. Thanks for all the help and clarification. push $0x7b // 123 in decimal push $0x0 -- Manavendra Nath Manav -- To unsubscribe from this list: send the line unsubscribe linux-kernel in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: Why exported const value modified by another driver not updated in original driver
On Tue, Sep 04, 2012 at 03:58:20PM +0530, Manavendra Nath Manav wrote: > Is the above a genuine kernel bug, or i am missing something out here. Pls > help. > When you declare something as const then the compiler assumes it really is const and uses a literal instead of reading from memory. I'm surprised the compiler doesn't print a warning message. It has to do with compilers, nothing to do with kernels. regards, dan carpenter -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: Why exported const value modified by another driver not updated in original driver
Hi! On Die, 2012-09-04 at 15:00 +0530, Manavendra Nath Manav wrote: [] > I have declared a static const int variable in one driver and exported > that variable symbol. Exporting a "static" variable makes conceptually no sense whatsoever - either you want it exported (so you should remove the "static") or you want it not exported - which is precisely the definition of "static". > 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's a bug in your drivers. The C compiler sees an initialized "static const" variable. So the C compiler knows that it is not accessible from other compilation units (because it is "static") and the variable isn't changing within the current compilation unit (because the variable as such is "const" and their is no non-const pointer to it or similar). Since this is the only compilation unit, it knows that no one will/can change it's value. So the C compiler happily replaces all references to the variable with it's value - just as if you would have #define'd that value. Obviously, later on it is exported and the variable actually exists and you can change it in the other file. But since the value has been replaced literally by "123" at compile (more precise: optimizer time), you don't see a change in the printk()s. > Interestingly, when i change the declaration of variable to "volatile" > in driver.c as "static const volatile int" then the driver.ko prints > modified value "987" during rmmod and the original value "123" is > lost. What is the difference between this and previous declaration? "volatile" tells the C compiler that a variable may change it's value without explicit code. A typical use case are hardware registers which are changed by the underlying hardware. In user space, variables may be modified by signal handlers (which can occur at any time in a program - unless specifcally blocked). So the optimizer must not accesses to "volatile" variables because they may change their value at any time. > Please forgive if you find this question silly? That all is actually pure (K)C and has nothing to do with the kernel. > static const int value = 123; [...] > EXPORT_SYMBOL(value); I wonder if we can modify EXPORT_SYMBOL() so that it compile-time-fails for "static" variables. And if we actually want that. Bernd -- Bernd Petrovitsch Email : be...@petrovitsch.priv.at LUGA : http://www.luga.at -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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 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 >> 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: Why exported const value modified by another driver not updated in original driver
On Tue, 4 Sep 2012 15:00:16 +0530 Manavendra Nath Manav 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. > 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's const, why is it even a surprise. If you don't understand what is going on then I suggest you disassemble the code. That will I think let you understand what is going on. Alan -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: Why exported const value modified by another driver not updated in original driver
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 > 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/. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: Why exported const value modified by another driver not updated in original driver
On Tue, Sep 4, 2012 at 3:00 PM, Manavendra Nath Manav
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.
> 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?
>
> [root@localhost bug]# uname -a
> Linux localhost.localdomain 3.5.3 #3 SMP Mon Sep 3 21:52:12 IST 2012
> i686 i686 i386 GNU/Linux
>
>
> [root@localhost bug]# cat driver.c
> #include
>
> static const int value = 123;
>
> int init_module()
> {
> printk("Base driver (init): value = %d\n", value);
> printk("Base driver (init): virtual address of value = %p\n", (void
> *));
> printk("Base driver (init): physical address of value = %p\n", (void
> *)__pa());
> return 0;
> }
>
> void cleanup_module()
> {
> printk("Base driver (exit): value = %d\n", value);
> printk("Base driver (exit): virtual address of value = %p\n", (void
> *));
> printk("Base driver (exit): physical address of value = %p\n", (void
> *)__pa());
> }
> EXPORT_SYMBOL(value);
>
> ==
> [root@localhost bug]# cat hacker.c
> #include
>
> extern int value;
>
> int init_module()
> {
> value = 987;
> printk("Hacker driver (init): value = %d\n",
> value);
> printk("Hacker Base driver (init): virtual address of value = %p\n",
> (void *));
> printk("Hacker Base driver (init): physical address of value = %p\n",
> (void *)__pa());
> return 0;
> }
>
> void cleanup_module()
> {
> printk("Hacker driver (exit): value = %d\n",
> value);
> printk("Hacker driver (exit): virtual address of value = %p\n",
> (void *));
> printk("Hacker driver (exit): physical address of value = %p\n",
> (void *)__pa());
> return;
> }
>
> =
> [root@localhost bug]# insmod driver.ko
> Base driver (init): value = 123
> Base driver (init): virtual address of value = f89d61c8
> Base driver (init): physical address of value = 389d61c8
>
> [root@localhost bug]# insmod hacker.ko
> Hacker driver (init): value = 987
> Hacker Base driver (init): virtual address of value = f89d61c8
> Hacker Base driver (init): physical address of value = 389d61c8
>
> [root@localhost bug]# rmmod hacker.ko
> Hacker driver (exit): value = 987
> Hacker driver (exit): virtual address of value = f89d61c8
> Hacker driver (exit): physical address of value = 389d61c8
>
> [root@localhost bug]# rmmod driver.ko
> Base driver (exit): value = 123
> Base driver (exit): virtual address of value = f89d61c8
> Base driver (exit): physical address of value = 389d61c8
>
> [root@localhost bug]# cat /proc/kallsyms | grep value
> f89f91c8 R value[driver]
> f89f9088 r __ksymtab_value [driver]
> f89f91cc r __kstrtab_value [driver]
>
> Interestingly, when i change the declaration of variable to "volatile"
> in driver.c as "static const volatile int" then the driver.ko prints
> modified value "987" during rmmod and the original value "123" is
> lost. What is the difference between this and previous declaration?
> Please forgive if you find this question silly?
> --
> Manavendra Nath Manav
Is the above a genuine kernel bug, or i am missing something out here. Pls help.
Thanks,
Manavendra Nath Manav
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
Why exported const value modified by another driver not updated in original driver
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.
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?
[root@localhost bug]# uname -a
Linux localhost.localdomain 3.5.3 #3 SMP Mon Sep 3 21:52:12 IST 2012
i686 i686 i386 GNU/Linux
[root@localhost bug]# cat driver.c
#include
static const int value = 123;
int init_module()
{
printk("Base driver (init): value = %d\n", value);
printk("Base driver (init): virtual address of value = %p\n", (void
*));
printk("Base driver (init): physical address of value = %p\n", (void
*)__pa());
return 0;
}
void cleanup_module()
{
printk("Base driver (exit): value = %d\n", value);
printk("Base driver (exit): virtual address of value = %p\n", (void
*));
printk("Base driver (exit): physical address of value = %p\n", (void
*)__pa());
}
EXPORT_SYMBOL(value);
==
[root@localhost bug]# cat hacker.c
#include
extern int value;
int init_module()
{
value = 987;
printk("Hacker driver (init): value = %d\n", value);
printk("Hacker Base driver (init): virtual address of value = %p\n",
(void *));
printk("Hacker Base driver (init): physical address of value = %p\n",
(void *)__pa());
return 0;
}
void cleanup_module()
{
printk("Hacker driver (exit): value = %d\n", value);
printk("Hacker driver (exit): virtual address of value = %p\n",
(void *));
printk("Hacker driver (exit): physical address of value = %p\n",
(void *)__pa());
return;
}
=
[root@localhost bug]# insmod driver.ko
Base driver (init): value = 123
Base driver (init): virtual address of value = f89d61c8
Base driver (init): physical address of value = 389d61c8
[root@localhost bug]# insmod hacker.ko
Hacker driver (init): value = 987
Hacker Base driver (init): virtual address of value = f89d61c8
Hacker Base driver (init): physical address of value = 389d61c8
[root@localhost bug]# rmmod hacker.ko
Hacker driver (exit): value = 987
Hacker driver (exit): virtual address of value = f89d61c8
Hacker driver (exit): physical address of value = 389d61c8
[root@localhost bug]# rmmod driver.ko
Base driver (exit): value = 123
Base driver (exit): virtual address of value = f89d61c8
Base driver (exit): physical address of value = 389d61c8
[root@localhost bug]# cat /proc/kallsyms | grep value
f89f91c8 R value[driver]
f89f9088 r __ksymtab_value [driver]
f89f91cc r __kstrtab_value [driver]
Interestingly, when i change the declaration of variable to "volatile"
in driver.c as "static const volatile int" then the driver.ko prints
modified value "987" during rmmod and the original value "123" is
lost. What is the difference between this and previous declaration?
Please forgive if you find this question silly?
--
Manavendra Nath Manav
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
Why exported const value modified by another driver not updated in original driver
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.
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?
[root@localhost bug]# uname -a
Linux localhost.localdomain 3.5.3 #3 SMP Mon Sep 3 21:52:12 IST 2012
i686 i686 i386 GNU/Linux
[root@localhost bug]# cat driver.c
#include linux/module.h
static const int value = 123;
int init_module()
{
printk(Base driver (init): value = %d\n, value);
printk(Base driver (init): virtual address of value = %p\n, (void
*)value);
printk(Base driver (init): physical address of value = %p\n, (void
*)__pa(value));
return 0;
}
void cleanup_module()
{
printk(Base driver (exit): value = %d\n, value);
printk(Base driver (exit): virtual address of value = %p\n, (void
*)value);
printk(Base driver (exit): physical address of value = %p\n, (void
*)__pa(value));
}
EXPORT_SYMBOL(value);
==
[root@localhost bug]# cat hacker.c
#include linux/module.h
extern int value;
int init_module()
{
value = 987;
printk(Hacker driver (init): value = %d\n, value);
printk(Hacker Base driver (init): virtual address of value = %p\n,
(void *)value);
printk(Hacker Base driver (init): physical address of value = %p\n,
(void *)__pa(value));
return 0;
}
void cleanup_module()
{
printk(Hacker driver (exit): value = %d\n, value);
printk(Hacker driver (exit): virtual address of value = %p\n,
(void *)value);
printk(Hacker driver (exit): physical address of value = %p\n,
(void *)__pa(value));
return;
}
=
[root@localhost bug]# insmod driver.ko
Base driver (init): value = 123
Base driver (init): virtual address of value = f89d61c8
Base driver (init): physical address of value = 389d61c8
[root@localhost bug]# insmod hacker.ko
Hacker driver (init): value = 987
Hacker Base driver (init): virtual address of value = f89d61c8
Hacker Base driver (init): physical address of value = 389d61c8
[root@localhost bug]# rmmod hacker.ko
Hacker driver (exit): value = 987
Hacker driver (exit): virtual address of value = f89d61c8
Hacker driver (exit): physical address of value = 389d61c8
[root@localhost bug]# rmmod driver.ko
Base driver (exit): value = 123
Base driver (exit): virtual address of value = f89d61c8
Base driver (exit): physical address of value = 389d61c8
[root@localhost bug]# cat /proc/kallsyms | grep value
f89f91c8 R value[driver]
f89f9088 r __ksymtab_value [driver]
f89f91cc r __kstrtab_value [driver]
Interestingly, when i change the declaration of variable to volatile
in driver.c as static const volatile int then the driver.ko prints
modified value 987 during rmmod and the original value 123 is
lost. What is the difference between this and previous declaration?
Please forgive if you find this question silly?
--
Manavendra Nath Manav
--
To unsubscribe from this list: send the line unsubscribe linux-kernel in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
Re: Why exported const value modified by another driver not updated in original driver
On Tue, Sep 4, 2012 at 3:00 PM, Manavendra Nath Manav
mnm.ker...@gmail.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.
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?
[root@localhost bug]# uname -a
Linux localhost.localdomain 3.5.3 #3 SMP Mon Sep 3 21:52:12 IST 2012
i686 i686 i386 GNU/Linux
[root@localhost bug]# cat driver.c
#include linux/module.h
static const int value = 123;
int init_module()
{
printk(Base driver (init): value = %d\n, value);
printk(Base driver (init): virtual address of value = %p\n, (void
*)value);
printk(Base driver (init): physical address of value = %p\n, (void
*)__pa(value));
return 0;
}
void cleanup_module()
{
printk(Base driver (exit): value = %d\n, value);
printk(Base driver (exit): virtual address of value = %p\n, (void
*)value);
printk(Base driver (exit): physical address of value = %p\n, (void
*)__pa(value));
}
EXPORT_SYMBOL(value);
==
[root@localhost bug]# cat hacker.c
#include linux/module.h
extern int value;
int init_module()
{
value = 987;
printk(Hacker driver (init): value = %d\n,
value);
printk(Hacker Base driver (init): virtual address of value = %p\n,
(void *)value);
printk(Hacker Base driver (init): physical address of value = %p\n,
(void *)__pa(value));
return 0;
}
void cleanup_module()
{
printk(Hacker driver (exit): value = %d\n,
value);
printk(Hacker driver (exit): virtual address of value = %p\n,
(void *)value);
printk(Hacker driver (exit): physical address of value = %p\n,
(void *)__pa(value));
return;
}
=
[root@localhost bug]# insmod driver.ko
Base driver (init): value = 123
Base driver (init): virtual address of value = f89d61c8
Base driver (init): physical address of value = 389d61c8
[root@localhost bug]# insmod hacker.ko
Hacker driver (init): value = 987
Hacker Base driver (init): virtual address of value = f89d61c8
Hacker Base driver (init): physical address of value = 389d61c8
[root@localhost bug]# rmmod hacker.ko
Hacker driver (exit): value = 987
Hacker driver (exit): virtual address of value = f89d61c8
Hacker driver (exit): physical address of value = 389d61c8
[root@localhost bug]# rmmod driver.ko
Base driver (exit): value = 123
Base driver (exit): virtual address of value = f89d61c8
Base driver (exit): physical address of value = 389d61c8
[root@localhost bug]# cat /proc/kallsyms | grep value
f89f91c8 R value[driver]
f89f9088 r __ksymtab_value [driver]
f89f91cc r __kstrtab_value [driver]
Interestingly, when i change the declaration of variable to volatile
in driver.c as static const volatile int then the driver.ko prints
modified value 987 during rmmod and the original value 123 is
lost. What is the difference between this and previous declaration?
Please forgive if you find this question silly?
--
Manavendra Nath Manav
Is the above a genuine kernel bug, or i am missing something out here. Pls help.
Thanks,
Manavendra Nath Manav
--
To unsubscribe from this list: send the line unsubscribe linux-kernel in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
Re: Why exported const value modified by another driver not updated in original driver
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.ker...@gmail.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/. -- To unsubscribe from this list: send the line unsubscribe linux-kernel in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: Why exported const value modified by another driver not updated in original driver
On Tue, 4 Sep 2012 15:00:16 +0530 Manavendra Nath Manav mnm.ker...@gmail.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. 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's const, why is it even a surprise. If you don't understand what is going on then I suggest you disassemble the code. That will I think let you understand what is going on. Alan -- To unsubscribe from this list: send the line unsubscribe linux-kernel in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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 j...@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.ker...@gmail.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 -- To unsubscribe from this list: send the line unsubscribe linux-kernel in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: Why exported const value modified by another driver not updated in original driver
Hi! On Die, 2012-09-04 at 15:00 +0530, Manavendra Nath Manav wrote: [] I have declared a static const int variable in one driver and exported that variable symbol. Exporting a static variable makes conceptually no sense whatsoever - either you want it exported (so you should remove the static) or you want it not exported - which is precisely the definition of static. 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's a bug in your drivers. The C compiler sees an initialized static const variable. So the C compiler knows that it is not accessible from other compilation units (because it is static) and the variable isn't changing within the current compilation unit (because the variable as such is const and their is no non-const pointer to it or similar). Since this is the only compilation unit, it knows that no one will/can change it's value. So the C compiler happily replaces all references to the variable with it's value - just as if you would have #define'd that value. Obviously, later on it is exported and the variable actually exists and you can change it in the other file. But since the value has been replaced literally by 123 at compile (more precise: optimizer time), you don't see a change in the printk()s. Interestingly, when i change the declaration of variable to volatile in driver.c as static const volatile int then the driver.ko prints modified value 987 during rmmod and the original value 123 is lost. What is the difference between this and previous declaration? volatile tells the C compiler that a variable may change it's value without explicit code. A typical use case are hardware registers which are changed by the underlying hardware. In user space, variables may be modified by signal handlers (which can occur at any time in a program - unless specifcally blocked). So the optimizer must not accesses to volatile variables because they may change their value at any time. Please forgive if you find this question silly? That all is actually pure (KR-)C and has nothing to do with the kernel. static const int value = 123; [...] EXPORT_SYMBOL(value); I wonder if we can modify EXPORT_SYMBOL() so that it compile-time-fails for static variables. And if we actually want that. Bernd -- Bernd Petrovitsch Email : be...@petrovitsch.priv.at LUGA : http://www.luga.at -- To unsubscribe from this list: send the line unsubscribe linux-kernel in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: Why exported const value modified by another driver not updated in original driver
On Tue, Sep 04, 2012 at 03:58:20PM +0530, Manavendra Nath Manav wrote: Is the above a genuine kernel bug, or i am missing something out here. Pls help. When you declare something as const then the compiler assumes it really is const and uses a literal instead of reading from memory. I'm surprised the compiler doesn't print a warning message. It has to do with compilers, nothing to do with kernels. regards, dan carpenter -- To unsubscribe from this list: send the line unsubscribe linux-kernel in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
