Re: String literals in __init functions

[Mason]

On 25/03/2015 19:01, Joe Perches wrote:

> On Wed, 2015-03-25 at 18:56 +0100, Mason wrote:
>
> > AFAIU, functions only used at system init are tagged __init to have
> > the linker store them in a separate .init.text section, so memory can
> > be reclaimed once initialization is complete. Is that correct?
> >
> > The corresponding tag for data is __initdata (section .init.data)
> >
> > I started wondering if the string literals used in an __init functions
> > were automatically marked __initdata.
> >
> > Looking at the objdump output, I see that the string literals are,
> > in fact, stored in the .rodata section. I suppose that .rodata is NOT
> > reclaimed after init?
> >
> > This way seems to work:
> >
> > static       char XyZa[] __initdata  = KERN_ALERT "foo";
> > static const char XyZb[] __initconst = KERN_ALERT "bar";
> > void __init XyZc(void) { printk(XyZa); printk(XyZb); }
> >
> > $ arm-linux-gnueabihf-objdump -xd arch/arm/mach-tangox/time.o | grep XyZ
> > 00000000 l     O .init.data	00000006 XyZa
> > 00000000 l     O .init.rodata	00000006 XyZb
> > 00000000 g     F .init.text	00000028 XyZc
> > 00000000 <XyZc>:
> >
> > $ arm-linux-gnueabihf-objdump -xd vmlinux | grep XyZ
> > c021e360 l     O .init.data	00000006 XyZa
> > c0220090 l     O .init.data	00000006 XyZb
> > c020d928 g     F .init.text	00000028 XyZc
> > c020d928 <XyZc>:
> >
> > c020d928 <XyZc>:
> > c020d928:       e1a0c00d        mov     ip, sp
> > c020d92c:       e92dd800        push    {fp, ip, lr, pc}
> > c020d930:       e24cb004        sub     fp, ip, #4
> > c020d934:       e30e0360        movw    r0, #58208      ; 0xe360
> > c020d938:       e34c0021        movt    r0, #49185      ; 0xc021
> > c020d93c:       ebfe00c9        bl      c018dc68 <printk>
> > c020d940:       e3000090        movw    r0, #144        ; 0x90
> > c020d944:       e34c0022        movt    r0, #49186      ; 0xc022
> > c020d948:       ebfe00c6        bl      c018dc68 <printk>
> > c020d94c:       e89da800        ldm     sp, {fp, sp, pc}
> >
> > Did I miss something in init.h?
> > Or should it be done like above to reclaim string literals?
>
> No, you didn't miss anything.
>
> One proposal:
>
> https://lkml.org/lkml/2014/8/21/255

Thanks for the link!

Here's the equivalent gmane link for my own reference:
http://thread.gmane.org/gmane.linux.kernel/1771969

Basically, if I understand correctly, Ingo NAKed the patch, saying
this should be done automatically by the toolchain. That would make
for an interesting side-project...

For the record, I wrote a trivial wrapper for my limited use-case.

#define printk_init(format, ...) do { \
  static char fmt[] __initdata = format; printk(fmt, ## __VA_ARGS__); \
} while(0)

(I dislike the "statement-in-expression" extension, because vim thinks
there's a syntax error, and flashes a bright red block.)

https://gcc.gnu.org/onlinedocs/cpp/Variadic-Macros.html
https://gcc.gnu.org/onlinedocs/gcc/Statement-Exprs.html

Regards.


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