[PATCH v2 14/20] arm64: percpu: Implement preemptible read/write ops
David Laight
david.laight.linux at gmail.com
Wed Aug 5 05:08:48 PDT 2026
On Wed, 5 Aug 2026 10:24:04 +0100
Ryan Roberts <ryan.roberts at arm.com> wrote:
> 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 <mark.rutland at arm.com>
> > Cc: Ada Couprie Diaz <ada.coupriediaz at arm.com>
> > Cc: Ard Biesheuvel <ardb at kernel.org>
> > Cc: Catalin Marinas <catalin.marinas at arm.com>
> > Cc: James Morse <james.morse at arm.com>
> > Cc: Jinjie Ruan <ruanjinjie at huawei.com>
> > Cc: Marc Zyngier <maz at kernel.org>
> > Cc: Peter Zijlstra <peterz at infradead.org>
> > Cc: Vladimir Murzin <vladimir.murzin at arm.com>
> > Cc: Will Deacon <will at kernel.org>
> > Cc: Yang Shi <yang at os.amperecomputing.com>
> > ---
> 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.
It'll be the earlier patch that changed the casts.
The code is doing:
this_cpu_write(tr->array_buffer.data->ftrace_ignore_pid,
FTRACE_PID_IGNORE);
where the constant is -1.
I suspect the warning messages are coming from the switch case that are
optimised away because the size if wrong (ftrace_ignore_pid seems to
be a pid number to ignore and is 32 bits.
That does seem a long-winded way to access per-cpu data.
To make any sense the code must be running with preemption disabled and
be caching info in per-cpu memory.
In which case it can just access it directly.
OTOH it could save the 'global' address of the per-cpu data and then access
it using the pointer so that access would be a normal one.
David
>
> 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<---
>
>
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