On Monday, June 13, 2016 6:05:22 PM CEST Arnd Bergmann wrote:
> On Friday, June 10, 2016 11:33:07 AM CEST Jason Baron wrote:
> > On 06/10/2016 05:54 AM, Arnd Bergmann wrote:
> > > On Friday, May 20, 2016 5:16:36 PM CEST Jason Baron wrote:
> > >> Although dynamic debug is often only used for debug builds, sometimes its
> > >> enabled for production builds as well. Minimize its impact by using jump
> > >> labels. This reduces the text section by 7000+ bytes in the kernel image
> > >> below. It does increase data, but this should only be referenced when
> > >> changing the direction of the branches, and hence usually not in cache.
> > >>
> > >> text data bss dec hex filename
> > >> 8194852 4879776 925696 14000324 d5a0c4 vmlinux.pre
> > >> 8187337 4960224 925696 14073257 d6bda9 vmlinux.post
> > >>
> > >> Signed-off-by: Jason Baron <jba...@akamai.com>
> > >> ---
> > >
> > > This causes problems for some of my randconfig builds, when a dynamic
> > > debug call is used inside of an __exit function:
> > >
> > > `.exit.text' referenced in section `__jump_table' of drivers/built-in.o:
> > > defined in discarded section `.exit.text' of drivers/built-in.o
> > > `.exit.text' referenced in section `__jump_table' of drivers/built-in.o:
> > > defined in discarded section `.exit.text' of drivers/built-in.o
> > >
> >
> > I stuck pr_debug() in a few functions marked with __exit, but did not
> > reproduce yet. Can you share your .config and gcc --version.
> >
>
> I found these on ARM randconfig builds e.g. this one
> http://pastebin.com/raw/KjWHxnwU
>
> I also have some other patches applied that could have interacted with your
> change, so if you can't reproduce it easily, let me try it on a plain
> linux-next
> kernel.
>
> The compiler I use is arm-linux-gnueabi-gcc (GCC) 6.0.0 20160323
> (experimental)
Update: on ARM, I have been able to reproduce this with gcc-4.6
and gcc-4.8, so I'm pretty confident that this is independent of the
toolchain. However, I have so far failed to reproduce this on x86.
Looking at the exit_ceph() function, I get these two assembly outputs,
ARM fails with the link error above:
.section .exit.text,"ax",%progbits
.align 2
.syntax unified
.arm
.fpu softvfp
.type exit_ceph, %function
exit_ceph:
@ args = 0, pretend = 0, frame = 0
@ frame_needed = 1, uses_anonymous_args = 0
mov ip, sp @,
push {fp, ip, lr, pc} @
sub fp, ip, #4 @,,
sub sp, sp, #8 @,,
.syntax divided
@ 13 "/git/arm-soc/arch/arm/include/asm/jump_label.h" 1
1:
nop
.pushsection __jump_table, "aw"
.word 1b, .L341, descriptor.39418+20 @,
.popsection
@ 0 "" 2
.syntax unified
.L342:
ldr r0, .L344 @,
bl unregister_filesystem @
bl ceph_xattr_exit @
bl destroy_caches @
b .L343 @
.L341:
mov r1, #29 @,
ldr r0, .L344+4 @,
bl ceph_file_part @
mov r3, #1072 @ tmp118,
mov r2, #3 @,
stm sp, {r0, r3} @,,
ldr r1, .L344+8 @,
ldr r3, .L344+12 @,
ldr r0, .L344+16 @,
bl __dynamic_pr_debug @
b .L342 @
.L343:
sub sp, fp, #12 @,,
ldm sp, {fp, sp, pc} @
.L345:
.align 2
.L344:
.word .LANCHOR2+224
.word .LC0
.word .LC69
.word .LC1
.word .LANCHOR0+1088
.size exit_ceph, .-exit_ceph
and x86 has no link error with:
.type exit_ceph, @function
exit_ceph:
pushq %rbp #
movq %rsp, %rbp #,
#APP
# 35 "/git/arm-soc/arch/x86/include/asm/jump_label.h" 1
1:.byte 0x0f,0x1f,0x44,0x00,0
.pushsection __jump_table, "aw"
.balign 8
.quad 1b, .L350, descriptor.39765+40 + 0 #,,
.popsection
# 0 "" 2
#NO_APP
.L351:
movq $ceph_fs_type, %rdi #,
call unregister_filesystem #
call ceph_xattr_exit #
call destroy_caches #
popq %rbp #
ret
.L350:
movl $29, %esi #,
movq $.LC0, %rdi #,
call ceph_file_part #
movl $1072, %r9d #,
movq %rax, %r8 #, D.41790
movq $.LC1, %rcx #,
movl $3, %edx #,
movq $.LC85, %rsi #,
movq $descriptor.39765, %rdi #,
call __dynamic_pr_debug #
jmp .L351 #
.size exit_ceph, .-exit_ceph
In both cases, the __jump_table section clearly has a reference to a
discarded section.
Arnd
