Re: [PATCH v2 14/20] arm64: percpu: Implement preemptible read/write ops
Ryan Roberts <[email protected]>
| Newsgroups | org.infradead.lists.linux-arm-kernel,org.kernel.vger.stable |
|---|---|
| Message-ID | <[email protected]> |
On 04/08/2026 18:04, Mark Rutland wrote:
> Use the PCPU GPR infrastructure to implement preemptible this_cpu_read()
> and this_cpu_write().
>
> This change means that this_cpu_read() will always use a plain LDR, even
> in LTO configurations where READ_ONCE() will use LDA[P]R. Using plain
> LDR is preferable, given that the rationale for using LDA[P]R in
> READ_ONCE() was to retain address dependencies against values written by
> other CPUs, which isn't expected usage for this_cpu_read(). Using plain
> LDR will enforce fewer ordering constraints.
>
> Test case:
>
> | void outline_this_cpu_write_u64(u64 __percpu *p, u64 v)
> | {
> | this_cpu_write(*p, v);
> | }
>
> Generated code before this patch (v7.2-rc4):
>
> | <outline_this_cpu_write_u64>:
> | paciasp
> | stp x29, x30, [sp, #-16]!
> | mrs x2, sp_el0
> | mov x29, sp
> | ldr w3, [x2, #8]
> | add w3, w3, #0x1
> | str w3, [x2, #8]
> | mrs x3, tpidr_el1
> | str x1, [x0, x3]
> | ldr x0, [x2, #8]
> | sub x0, x0, #0x1
> | str w0, [x2, #8]
> | cbz x0, 1f
> | ldr x0, [x2, #8]
> | cbnz x0, 2f
> | 1: bl preempt_schedule_notrace
> | 2: ldp x29, x30, [sp], #16
> | autiasp
> | ret
>
> Generated code after this patch:
>
> | <outline_this_cpu_write_u64>:
> | mrs x2, sp_el0
> | mov w3, #0x7c60
> | strh w3, [x2, #20]
> | mrs x3, tpidr_el1
> | str x1, [x0, x3]
> | strh wzr, [x2, #20]
> | ret
>
> Signed-off-by: Mark Rutland <[email protected]>
> Cc: Ada Couprie Diaz <[email protected]>
> Cc: Ard Biesheuvel <[email protected]>
> Cc: Catalin Marinas <[email protected]>
> Cc: James Morse <[email protected]>
> Cc: Jinjie Ruan <[email protected]>
> Cc: Marc Zyngier <[email protected]>
> Cc: Peter Zijlstra <[email protected]>
> Cc: Vladimir Murzin <[email protected]>
> Cc: Will Deacon <[email protected]>
> Cc: Yang Shi <[email protected]>
> ---
FYI I'm seeing build warnings caused by this patch (with ftrace enabled - based
on the warnings, I'm guessing that's the key bit), using:
aarch64-linux-gnu-gcc (Debian 14.2.0-19) 14.2.0, GNU ld (GNU Binutils for Debian) 2.44
I haven't investigated the cause.
Thanks,
Ryan
---8<---
In file included from linux/arch/arm64/include/asm/spectre.h:17,
from linux/arch/arm64/include/asm/processor.h:47,
from linux/include/linux/sched.h:13,
from linux/include/linux/ratelimit.h:6,
from linux/include/linux/dev_printk.h:16,
from linux/include/linux/device.h:15,
from linux/include/linux/node.h:18,
from linux/include/linux/cpu.h:17,
from linux/include/linux/stop_machine.h:5,
from linux/kernel/trace/ftrace.c:17:
linux/kernel/trace/ftrace.c: In function 'ftrace_filter_pid_sched_switch_probe':
linux/arch/arm64/include/asm/percpu.h:295:42: warning: conversion from 'long unsigned int' to 'u8' {aka 'unsigned char'} changes value from '18446744073709551615' to '255' [-Woverflow]
295 | _pcp_wrap(__percpu_write_8, pcp, (unsigned long)(val))
| ^~~~~~~~~~~~~~~~~~~~
linux/arch/arm64/include/asm/percpu.h:277:20: note: in definition of macro '_pcp_wrap'
277 | op(&(pcp), __VA_ARGS__); \
| ^~~~~~~~~~~
linux/include/linux/percpu-defs.h:369:25: note: in expansion of macro 'this_cpu_write_1'
369 | case 1: stem##1(variable, __VA_ARGS__);break; \
| ^~~~
linux/include/linux/percpu-defs.h:500:41: note: in expansion of macro '__pcpu_size_call'
500 | #define this_cpu_write(pcp, val) __pcpu_size_call(this_cpu_write_, pcp, val)
| ^~~~~~~~~~~~~~~~
linux/kernel/trace/ftrace.c:8628:17: note: in expansion of macro 'this_cpu_write'
8628 | this_cpu_write(tr->array_buffer.data->ftrace_ignore_pid,
| ^~~~~~~~~~~~~~
linux/arch/arm64/include/asm/percpu.h:297:43: warning: conversion from 'long unsigned int' to 'u16' {aka 'short unsigned int'} changes value from '18446744073709551615' to '65535' [-Woverflow]
297 | _pcp_wrap(__percpu_write_16, pcp, (unsigned long)(val))
| ^~~~~~~~~~~~~~~~~~~~
linux/arch/arm64/include/asm/percpu.h:277:20: note: in definition of macro '_pcp_wrap'
277 | op(&(pcp), __VA_ARGS__); \
| ^~~~~~~~~~~
linux/include/linux/percpu-defs.h:370:25: note: in expansion of macro 'this_cpu_write_2'
370 | case 2: stem##2(variable, __VA_ARGS__);break; \
| ^~~~
linux/include/linux/percpu-defs.h:500:41: note: in expansion of macro '__pcpu_size_call'
500 | #define this_cpu_write(pcp, val) __pcpu_size_call(this_cpu_write_, pcp, val)
| ^~~~~~~~~~~~~~~~
linux/kernel/trace/ftrace.c:8628:17: note: in expansion of macro 'this_cpu_write'
8628 | this_cpu_write(tr->array_buffer.data->ftrace_ignore_pid,
| ^~~~~~~~~~~~~~
linux/arch/arm64/include/asm/percpu.h:299:43: warning: conversion from 'long unsigned int' to 'u32' {aka 'unsigned int'} changes value from '18446744073709551615' to '4294967295' [-Woverflow]
299 | _pcp_wrap(__percpu_write_32, pcp, (unsigned long)(val))
| ^~~~~~~~~~~~~~~~~~~~
linux/arch/arm64/include/asm/percpu.h:277:20: note: in definition of macro '_pcp_wrap'
277 | op(&(pcp), __VA_ARGS__); \
| ^~~~~~~~~~~
linux/include/linux/percpu-defs.h:371:25: note: in expansion of macro 'this_cpu_write_4'
371 | case 4: stem##4(variable, __VA_ARGS__);break; \
| ^~~~
linux/include/linux/percpu-defs.h:500:41: note: in expansion of macro '__pcpu_size_call'
500 | #define this_cpu_write(pcp, val) __pcpu_size_call(this_cpu_write_, pcp, val)
| ^~~~~~~~~~~~~~~~
linux/kernel/trace/ftrace.c:8628:17: note: in expansion of macro 'this_cpu_write'
8628 | this_cpu_write(tr->array_buffer.data->ftrace_ignore_pid,
| ^~~~~~~~~~~~~~
linux/kernel/trace/ftrace.c: In function 'ignore_task_cpu':
linux/arch/arm64/include/asm/percpu.h:295:42: warning: conversion from 'long unsigned int' to 'u8' {aka 'unsigned char'} changes value from '18446744073709551615' to '255' [-Woverflow]
295 | _pcp_wrap(__percpu_write_8, pcp, (unsigned long)(val))
| ^~~~~~~~~~~~~~~~~~~~
linux/arch/arm64/include/asm/percpu.h:277:20: note: in definition of macro '_pcp_wrap'
277 | op(&(pcp), __VA_ARGS__); \
| ^~~~~~~~~~~
linux/include/linux/percpu-defs.h:369:25: note: in expansion of macro 'this_cpu_write_1'
369 | case 1: stem##1(variable, __VA_ARGS__);break; \
| ^~~~
linux/include/linux/percpu-defs.h:500:41: note: in expansion of macro '__pcpu_size_call'
500 | #define this_cpu_write(pcp, val) __pcpu_size_call(this_cpu_write_, pcp, val)
| ^~~~~~~~~~~~~~~~
linux/kernel/trace/ftrace.c:8898:17: note: in expansion of macro 'this_cpu_write'
8898 | this_cpu_write(tr->array_buffer.data->ftrace_ignore_pid,
| ^~~~~~~~~~~~~~
linux/arch/arm64/include/asm/percpu.h:297:43: warning: conversion from 'long unsigned int' to 'u16' {aka 'short unsigned int'} changes value from '18446744073709551615' to '65535' [-Woverflow]
297 | _pcp_wrap(__percpu_write_16, pcp, (unsigned long)(val))
| ^~~~~~~~~~~~~~~~~~~~
linux/arch/arm64/include/asm/percpu.h:277:20: note: in definition of macro '_pcp_wrap'
277 | op(&(pcp), __VA_ARGS__); \
| ^~~~~~~~~~~
linux/include/linux/percpu-defs.h:370:25: note: in expansion of macro 'this_cpu_write_2'
370 | case 2: stem##2(variable, __VA_ARGS__);break; \
| ^~~~
linux/include/linux/percpu-defs.h:500:41: note: in expansion of macro '__pcpu_size_call'
500 | #define this_cpu_write(pcp, val) __pcpu_size_call(this_cpu_write_, pcp, val)
| ^~~~~~~~~~~~~~~~
linux/kernel/trace/ftrace.c:8898:17: note: in expansion of macro 'this_cpu_write'
8898 | this_cpu_write(tr->array_buffer.data->ftrace_ignore_pid,
| ^~~~~~~~~~~~~~
linux/arch/arm64/include/asm/percpu.h:299:43: warning: conversion from 'long unsigned int' to 'u32' {aka 'unsigned int'} changes value from '18446744073709551615' to '4294967295' [-Woverflow]
299 | _pcp_wrap(__percpu_write_32, pcp, (unsigned long)(val))
| ^~~~~~~~~~~~~~~~~~~~
linux/arch/arm64/include/asm/percpu.h:277:20: note: in definition of macro '_pcp_wrap'
277 | op(&(pcp), __VA_ARGS__); \
| ^~~~~~~~~~~
linux/include/linux/percpu-defs.h:371:25: note: in expansion of macro 'this_cpu_write_4'
371 | case 4: stem##4(variable, __VA_ARGS__);break; \
| ^~~~
linux/include/linux/percpu-defs.h:500:41: note: in expansion of macro '__pcpu_size_call'
500 | #define this_cpu_write(pcp, val) __pcpu_size_call(this_cpu_write_, pcp, val)
| ^~~~~~~~~~~~~~~~
linux/kernel/trace/ftrace.c:8898:17: note: in expansion of macro 'this_cpu_write'
8898 | this_cpu_write(tr->array_buffer.data->ftrace_ignore_pid,
| ^~~~~~~~~~~~~~
---8<---