Re: [PATCH RFC v2 2/2] arm64: vdso: Implement __vdso_futex_robust_try_unlock()

Thomas Weißschuh
2026-04-27T16:48:48Z

On 2026-04-27 13:26:41-0300, André Almeida wrote:
> 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@igalia.com>
> > > ---
> > > 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?

Are 64-bit processes supposed to have the list32 variant already?
If so, you need this function, *with the same name* in both vDSOs.
In any case for the 32-bit vDSO you'll need to extend the build system
to create a vdso32-offsets.h. If you have the list32 variant twice, use
differently named VDSO{,32}_ constants to refer to them from kernel code.

> > (...)
> >
> > > 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?

No, they are not UAPI. It should go into include/vdso/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?

arch/arm64/kernel/vdso/vfutex_robust_list_try_unlock.c:26:18: warning: value size does not match register size specified by the constraint and modifier [-Wasm-operand-widths]
26 | : [val] "=&r" (val), [result] "=r" (result)
| ^
arch/arm64/kernel/vdso/vfutex_robust_list_try_unlock.c:17:10: note: use constraint modifier "w"
17 | " ldxr %[val], %[lock] \n"
| ^~~~~~
| %w[val]
arch/arm64/kernel/vdso/vfutex_robust_list_try_unlock.c:27:16: warning: value size does not match register size specified by the constraint and modifier [-Wasm-operand-widths]
27 | : [tid] "r" (tid), [lock] "Q" (*lock), [pop] "Q" (*pop)
| ^
arch/arm64/kernel/vdso/vfutex_robust_list_try_unlock.c:18:9: note: use constraint modifier "w"
18 | " cmp %[tid], %[val] \n"
| ^~~~~~
| %w[tid]
arch/arm64/kernel/vdso/vfutex_robust_list_try_unlock.c:26:18: warning: value size does not match register size specified by the constraint and modifier [-Wasm-operand-widths]
26 | : [val] "=&r" (val), [result] "=r" (result)
| ^
arch/arm64/kernel/vdso/vfutex_robust_list_try_unlock.c:18:17: note: use constraint modifier "w"
18 | " cmp %[tid], %[val] \n"
| ^~~~~~
| %w[val]
arch/arm64/kernel/vdso/vfutex_robust_list_try_unlock.c:40:3: error: invalid operand in inline asm: '.globl __futex_list32_try_unlock_cs_start, __futex_list32_try_unlock_cs_success, __futex_list32_try_unlock_cs_end
prfm pstl1strm, $3
__futex_list32_try_unlock_cs_start:
ldxr ${0:w}, $3
cmp ${2:w}, ${0:w}
bne __futex_list32_try_unlock_cs_end
stlxr ${1:w}, wzr, ${3:w}
cbnz ${1:w}, __futex_list32_try_unlock_cs_start
__futex_list32_try_unlock_cs_success:
str wzr, ${4:w}
__futex_list32_try_unlock_cs_end:
'
40 | GLOBLS(32)
| ^
arch/arm64/kernel/vdso/vfutex_robust_list_try_unlock.c:7:20: note: expanded from macro 'GLOBLS'
7 | #define GLOBLS(sz) ".globl " LABEL(start, sz) ", " LABEL(success, sz) ", " LABEL(end, sz) "\n"
| ^
arch/arm64/kernel/vdso/vfutex_robust_list_try_unlock.c:40:3: error: invalid operand in inline asm: '.globl __futex_list32_try_unlock_cs_start, __futex_list32_try_unlock_cs_success, __futex_list32_try_unlock_cs_end
prfm pstl1strm, $3
__futex_list32_try_unlock_cs_start:
ldxr ${0:w}, $3
cmp ${2:w}, ${0:w}
bne __futex_list32_try_unlock_cs_end
stlxr ${1:w}, wzr, ${3:w}
cbnz ${1:w}, __futex_list32_try_unlock_cs_start
__futex_list32_try_unlock_cs_success:
str wzr, ${4:w}
__futex_list32_try_unlock_cs_end:
'
arch/arm64/kernel/vdso/vfutex_robust_list_try_unlock.c:7:20: note: expanded from macro 'GLOBLS'
7 | #define GLOBLS(sz) ".globl " LABEL(start, sz) ", " LABEL(success, sz) ", " LABEL(end, sz) "\n"
| ^
arch/arm64/kernel/vdso/vfutex_robust_list_try_unlock.c:46:4: error: unknown token in expression
46 | " stlxr %w[result], wzr, %w[lock] \n"
| ^
<inline asm>:7:19: note: instantiated into assembly here
7 | stlxr w9, wzr,
| ^
arch/arm64/kernel/vdso/vfutex_robust_list_try_unlock.c:46:4: error: invalid operand
46 | " stlxr %w[result], wzr, %w[lock] \n"
| ^
<inline asm>:7:19: note: instantiated into assembly here
7 | stlxr w9, wzr,
| ^
arch/arm64/kernel/vdso/vfutex_robust_list_try_unlock.c:49:4: error: unknown token in expression
49 | " str wzr, %w[pop] \n"
| ^
<inline asm>:10:14: note: instantiated into assembly here
10 | str wzr,
| ^
arch/arm64/kernel/vdso/vfutex_robust_list_try_unlock.c:49:4: error: invalid operand
49 | " str wzr, %w[pop] \n"
| ^
<inline asm>:10:14: note: instantiated into assembly here
10 | str wzr,
| ^
3 warnings and 6 errors generated.

> > > +
> > > + 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@igalia.com/

If the regular system calls don't currently support a 32-bit robust list
on 64-bit systems I am wondering why tglx added them to the x86_64 vDSO.
They seem pointless for now. Maybe to be ready for your series?

Also on x86_64, if wine WoW64 should end up using the 32-bit robust list
from a 64-bit process, the CONFIG_COMPAT dependency looks incorrect.

> > 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
>