[PATCH v3 2/5] arm64: vdso: Implement __vdso_futex_robust_try_unlock()
From: André Almeida
Date: Fri May 29 2026 - 12:43:23 EST
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@xxxxxxxxxx>
---
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/Kconfig b/arch/arm64/Kconfig
index 427151a9db7f..e10cb97a51c7 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -249,6 +249,7 @@ config ARM64
select HAVE_RELIABLE_STACKTRACE
select HAVE_POSIX_CPU_TIMERS_TASK_WORK
select HAVE_FUNCTION_ARG_ACCESS_API
+ select HAVE_FUTEX_ROBUST_UNLOCK
select MMU_GATHER_RCU_TABLE_FREE
select HAVE_RSEQ
select HAVE_RUST if RUSTC_SUPPORTS_ARM64
diff --git a/arch/arm64/include/asm/futex_robust.h b/arch/arm64/include/asm/futex_robust.h
new file mode 100644
index 000000000000..26b4006c4aeb
--- /dev/null
+++ b/arch/arm64/include/asm/futex_robust.h
@@ -0,0 +1,20 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_ARM64_FUTEX_ROBUST_H
+#define _ASM_ARM64_FUTEX_ROBUST_H
+
+#include <asm/ptrace.h>
+
+static __always_inline void __user *arm64_futex_robust_unlock_get_pop(struct pt_regs *regs)
+{
+ /*
+ * If Z bit is set then the ll/sc cmpxchg succeeded and the pending op
+ * pointer needs to be cleared.
+ */
+ return (regs->user_regs.pstate & PSR_Z_BIT) ?
+ (void __user *) regs->user_regs.regs[2] : NULL;
+}
+
+#define arch_futex_robust_unlock_get_pop(regs) \
+ arm64_futex_robust_unlock_get_pop(regs)
+
+#endif /* _ASM_ARM64_FUTEX_ROBUST_H */
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
# 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/arch/arm64/kernel/vdso/vfutex.c b/arch/arm64/kernel/vdso/vfutex.c
new file mode 100644
index 000000000000..3561b869d8bb
--- /dev/null
+++ b/arch/arm64/kernel/vdso/vfutex.c
@@ -0,0 +1,34 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+#include <linux/stringify.h>
+#include <vdso/futex.h>
+
+#define LABEL(name, sz) __stringify(__futex_list##sz##_try_unlock_cs_##name)
+
+#define GLOBLS(sz) ".globl " LABEL(start, sz) ", " LABEL(success, sz) ", " LABEL(end, sz) "\n"
+
+__u32 __vdso_futex_robust_list64_try_unlock(__u32 *lock, __u32 tid, __u64 *pop)
+{
+ register __u64 *pop_reg asm("x2") = pop;
+ __u32 val, result;
+
+ asm volatile (
+ GLOBLS(64)
+ " prfm pstl1strm, %[lock] \n"
+ "retry: \n"
+ " ldxr %w[val], %[lock] \n"
+ " cmp %w[tid], %w[val] \n"
+ " bne " LABEL(end, 64)" \n"
+ " stlxr %w[result], wzr, %[lock] \n"
+ " cbnz %w[result], retry \n"
+ LABEL(start, 64)": \n"
+ LABEL(success, 64)": \n"
+ " str xzr, %[pop_reg] \n"
+ LABEL(end, 64)": \n"
+
+ : [val] "=&r" (val), [result] "=&r" (result)
+ : [tid] "r" (tid), [lock] "Q" (*lock), [pop_reg] "Q" (*pop_reg)
+ : "memory"
+ );
+
+ return val;
+}
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>
+
/**
* __vdso_futex_robust_list64_try_unlock - Try to unlock an uncontended robust futex
* with a 64-bit pending op pointer
--
2.54.0