Messages in this thread | | | From | Nick Desaulniers <> | Date | Tue, 2 Jan 2018 10:51:38 -0800 | Subject | correct section for cpu_tss_rw? |
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(emailing the folks listed from running `./scripts/get_maintainer.pl -f` on arch/x86/kernel/process.c, arch/x86/include/asm/processor.h, and include/linux/percpu-defs.h)
Clang emits the following warning:
arch/x86/kernel/process.c:50:11: warning: section does not match previous declaration [-Wsection] __visible DEFINE_PER_CPU_SHARED_ALIGNED(struct tss_struct, cpu_tss_rw) = { ^ ./include/linux/percpu-defs.h:144:2: note: expanded from macro 'DEFINE_PER_CPU_SHARED_ALIGNED' DEFINE_PER_CPU_SECTION(type, name, PER_CPU_SHARED_ALIGNED_SECTION) \ ^ ./include/linux/percpu-defs.h:104:2: note: expanded from macro 'DEFINE_PER_CPU_SECTION' __PCPU_ATTRS(sec) PER_CPU_DEF_ATTRIBUTES \ ^ ./include/linux/percpu-defs.h:49:26: note: expanded from macro '__PCPU_ATTRS' __percpu __attribute__((section(PER_CPU_BASE_SECTION sec))) \ ^ ./arch/x86/include/asm/processor.h:365:1: note: previous attribute is here DECLARE_PER_CPU_PAGE_ALIGNED(struct tss_struct, cpu_tss_rw); ^ ./include/linux/percpu-defs.h:159:2: note: expanded from macro 'DECLARE_PER_CPU_PAGE_ALIGNED' DECLARE_PER_CPU_SECTION(type, name, "..page_aligned") \ ^ ./include/linux/percpu-defs.h:101:9: note: expanded from macro 'DECLARE_PER_CPU_SECTION' extern __PCPU_ATTRS(sec) __typeof__(type) name ^ ./include/linux/percpu-defs.h:49:26: note: expanded from macro '__PCPU_ATTRS' __percpu __attribute__((section(PER_CPU_BASE_SECTION sec))) \ ^
it seems that from commit c482feefe1a ("x86/entry/64: Make cpu_entry_area.tss read-only") that cpu_tss_rw is declared but then defined in two different sections. (Though, it looks like this issue predates that commit).
In include/linux/percpu-defs.h, there's two pairs of macros:
DECLARE_PER_CPU_SHARED_ALIGNED DEFINE_PER_CPU_SHARED_ALIGNED DECLARE_PER_CPU_PAGE_ALIGNED DEFINE_PER_CPU_PAGE_ALIGNED
It seems that cpu_tss_rw is defined as SHARED_ALIGNED, but then declared as PAGE_ALIGNED. Should be an easy fix (that I'm happy to author), but what section *should* cpu_tss_rw be in (SHARED_ALIGNED or PAGE_ALIGNED)? That affects whether I fix the declaration or definition (and thus the .h or the .c file).
From the comment in arch/x86/kernel/process.c#50: 43 /* 44 * per-CPU TSS segments. Threads are completely 'soft' on Linux, 45 * no more per-task TSS's. The TSS size is kept cacheline-aligned 46 * so they are allowed to end up in the .data..cacheline_aligned 47 * section. Since TSS's are completely CPU-local, we want them 48 * on exact cacheline boundaries, to eliminate cacheline ping-pong. 49 */ 50 __visible DEFINE_PER_CPU_SHARED_ALIGNED(struct tss_struct, cpu_tss_rw) = {
I suspect that cache-line alignment is stricter than page alignment, so the declaration should be fixed, but I was not sure and wanted to check?
-- Thanks, ~Nick Desaulniers
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