Re: [PATCH RFC v2 2/2] arm64: vdso: Implement __vdso_futex_robust_try_unlock()
From: André Almeida
Date: Mon Apr 27 2026 - 12:40:13 EST
Em 26/04/2026 15:30, Thomas Weißschuh escreveu:
On 2026-04-24 15:56:01-0300, André Almeida wrote:
(...)
Signed-off-by: André Almeida <andrealmeid@xxxxxxxxxx>
---
RFC:
- Should I duplicate the explanation found in the x86 commit or can I just
point to it?
- Only LL/SC for now but I can add LSE later if this looks good
- It the objdump I see that op_pending is store at x2. But how stable is this,
how can I write it in a way that's always x2?
---
arch/arm64/Kconfig | 1 +
arch/arm64/include/asm/futex_robust.h | 35 +++++++++++++
arch/arm64/kernel/vdso/Makefile | 9 +++-
arch/arm64/kernel/vdso/vdso.lds.S | 4 ++
.../kernel/vdso/vfutex_robust_list_try_unlock.c | 59 ++++++++++++++++++++++
5 files changed, 107 insertions(+), 1 deletion(-)
What about the actual 32-bit vDSO in arch/arm64/kernel/vdso32/ ?
Right, I missed that. Then I should move __vdso_futex_robust_list32_try_unlock() to arch/arm64/kernel/vdso32/ right?
(...)
diff --git a/arch/arm64/kernel/vdso/vfutex_robust_list_try_unlock.c b/arch/arm64/kernel/vdso/vfutex_robust_list_try_unlock.c
new file mode 100644
index 000000000000..e8a8fb22a2fa
--- /dev/null
+++ b/arch/arm64/kernel/vdso/vfutex_robust_list_try_unlock.c
@@ -0,0 +1,59 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+#include <vdso/futex.h>
+#include <linux/stringify.h>
+
+#define LABEL(name, sz) __stringify(__futex_list##sz##_try_unlock_cs_##name)
We should have some defines for these symbols. While they are not
userspace ABI, they will be used by the selftests.
Do you mean to have this defined at include/uapi/linux/futex.h?
+#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)
+{
+ __u32 val, result;
+
+ asm volatile (
+ GLOBLS(64)
+ " prfm pstl1strm, %[lock] \n"
+ LABEL(start, 64)": \n"
+ " ldxr %[val], %[lock] \n"
+ " cmp %[tid], %[val] \n"
+ " bne " LABEL(end, 64)" \n"
+ " stlxr %w[result], xzr, %[lock] \n"
+ " cbnz %w[result], " LABEL(start, 64)" \n"
+ LABEL(success, 64)": \n"
+ " str xzr, %[pop] \n"
+ LABEL(end, 64)": \n"
+
+ : [val] "=&r" (val), [result] "=r" (result)
+ : [tid] "r" (tid), [lock] "Q" (*lock), [pop] "Q" (*pop)
+ : "memory"
+ );
My clang 22.1.3 chokes on the assembly in this patch.
Do you mind sharing the output?
+
+ return val;
+}
+
+#ifdef CONFIG_COMPAT_VDSO
I am wondering about the CONFIG_COMPAT{,_VDSO} dependency here.
As far as I know the list32 variant is meant to be used by code
emulators which run 32-bit code on a 64-bit kernel, for example FEX.
But these emulators don't actually seem to need CONFIG_COMPAT.
So the dependency does not look correct.
The space savings also should be irrelevant.
Right, good catch. In the new syscall I had to do something similar[1], to expose the 32-bit functions to 64-bit kernels as well, and not hide them behind CONFIG_COMPAT.
[1] https://lore.kernel.org/lkml/20251122-tonyk-robust_futex-v6-2-05fea005a0fd@xxxxxxxxxx/
The x86 series from Thomas does the same, maybe he will read this
comment, otherwise I'll bring it up on his series, too.
+__u32 __vdso_futex_robust_list32_try_unlock(__u32 *lock, __u32 tid, __u32 *pop)
+{
+ __u32 val, result;
+
+ asm volatile (
+ GLOBLS(32)
+ " prfm pstl1strm, %[lock] \n"
+ LABEL(start, 32)": \n"
+ " ldxr %w[val], %[lock] \n"
+ " cmp %w[tid], %w[val] \n"
+ " bne " LABEL(end, 32)" \n"
+ " stlxr %w[result], wzr, %w[lock] \n"
+ " cbnz %w[result], " LABEL(start, 32)" \n"
+ LABEL(success, 32)": \n"
+ " str wzr, %w[pop] \n"
+ LABEL(end, 32)": \n"
+
+ : [val] "=&r" (val), [result] "=r" (result)
+ : [tid] "r" (tid), [lock] "Q" (*lock), [pop] "Q" (*pop)
+ : "memory"
+ );
+
+ return val;
+}
+#endif