Re: [PATCH v3 2/5] arm64: vdso: Implement __vdso_futex_robust_try_unlock()

["Thomas Weißschuh" <linux@weissschuh.net>]

On 2026-05-29 13:33:54-0300, André Almeida wrote:
> Based on the x86 implementation, implement the vDSO function for unlocking
> a robust futex correctly.
> 
> Commit xxxxxxxxxxxx ("x86/vdso: Implement __vdso_futex_robust_try_unlock()") has
> the full explanation about why this mechanism is needed.
> 
> The unlock assembly sequence for arm64 is:
> 
> 	__vdso_futex_robust_list64_try_unlock:
> 	retry:
> 		ldxr	w8, [x0] // Load the value from *futex
> 		cmp	w1, w8   // Compare with TID
> 		b.ne	__vdso_futex_list64_try_unlock_cs_end
> 		stlxr	w9, wzr, [x0] // Try to zero *futex
> 		cbnz	w9, retry
> 	__vdso_futex_list64_try_unlock_cs_start:
> 		str	xzr, [x2] // After zeroing *futex, zero *op_pending
> 	__vdso_futex_list64_try_unlock_cs_end>:
> 
> The decision regarding if the pointer should be cleared or not lies on checking
> the condition flag zero:
> 
> 	return (regs->user_regs.pstate & PSR_Z_BIT) ?
> 		(void __user *) regs->user_regs.regs[2] : NULL;
> 
> If it's not zero, that means that the comparassion worked and the kernel should
> clear op_pending (if userspace didn't managed to) stored at x2.
> 
> Signed-off-by: André Almeida <andrealmeid@igalia.com>
> ---
> Notes:
>  - Only LL/SC for now but I can add LSE later if this looks good
> 
> v3:
>  - Managed to get pop to always be stored at x2
> ---
>  arch/arm64/Kconfig                    |  1 +
>  arch/arm64/include/asm/futex_robust.h | 20 ++++++++++++++++++++
>  arch/arm64/kernel/vdso/Makefile       |  9 ++++++++-
>  arch/arm64/kernel/vdso/vfutex.c       | 34 ++++++++++++++++++++++++++++++++++
>  include/vdso/futex.h                  |  1 +
>  5 files changed, 64 insertions(+), 1 deletion(-)

(...)

> diff --git a/arch/arm64/kernel/vdso/Makefile b/arch/arm64/kernel/vdso/Makefile
> index 7dec05dd33b7..3c7f220fe783 100644
> --- a/arch/arm64/kernel/vdso/Makefile
> +++ b/arch/arm64/kernel/vdso/Makefile
> @@ -9,7 +9,8 @@
>  # Include the generic Makefile to check the built vdso.
>  include $(srctree)/lib/vdso/Makefile.include
>  
> -obj-vdso := vgettimeofday.o note.o sigreturn.o vgetrandom.o vgetrandom-chacha.o
> +obj-vdso := vgettimeofday.o note.o sigreturn.o vgetrandom.o vgetrandom-chacha.o \
> +	    vfutex.o

Should depend on CONFIG_FUTEX_ROBUST_UNLOCK ?

>  
>  # Build rules
>  targets := $(obj-vdso) vdso.so vdso.so.dbg
> @@ -45,9 +46,11 @@ CC_FLAGS_ADD_VDSO := -O2 -mcmodel=tiny -fasynchronous-unwind-tables
>  
>  CFLAGS_REMOVE_vgettimeofday.o = $(CC_FLAGS_REMOVE_VDSO)
>  CFLAGS_REMOVE_vgetrandom.o = $(CC_FLAGS_REMOVE_VDSO)
> +CFLAGS_REMOVE_vfutex.o = $(CC_FLAGS_REMOVE_VDSO)
>  
>  CFLAGS_vgettimeofday.o = $(CC_FLAGS_ADD_VDSO)
>  CFLAGS_vgetrandom.o = $(CC_FLAGS_ADD_VDSO)
> +CFLAGS_vfutex.o = $(CC_FLAGS_ADD_VDSO)
>  
>  ifneq ($(c-gettimeofday-y),)
>    CFLAGS_vgettimeofday.o += -include $(c-gettimeofday-y)
> @@ -57,6 +60,10 @@ ifneq ($(c-getrandom-y),)
>    CFLAGS_vgetrandom.o += -include $(c-getrandom-y)
>  endif
>  
> +ifneq ($(c-futex-y),)
> +  CFLAGS_vfutex.o += -include $(c-futex-y)
> +endif
> +
>  targets += vdso.lds
>  CPPFLAGS_vdso.lds += -P -C -U$(ARCH)

(...)

> diff --git a/include/vdso/futex.h b/include/vdso/futex.h
> index 3cd175eefe64..80934561a10d 100644
> --- a/include/vdso/futex.h
> +++ b/include/vdso/futex.h
> @@ -4,6 +4,7 @@
>  
>  #include <uapi/linux/types.h>
>  
> +

Spurious change.

>  /**
>   * __vdso_futex_robust_list64_try_unlock - Try to unlock an uncontended robust futex
>   *					   with a 64-bit pending op pointer
> 
> -- 
> 2.54.0
>