Messages in this thread | | | Date | Thu, 26 Mar 2015 13:40:15 +0100 | From | Mason <> | Subject | Re: String literals in __init functions |
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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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