[PATCH RFC 0/2] arm64: vdso: Implement __vdso_futex_robust_try_unlock()

[André Almeida]

Hi folks,

This is my take on implementing the new vDSO for unlocking a robust futex in
arm64. If you don't know what's that, Thomas wrote a good summary,
including the motivation for this work and the x86 implementation:

   https://lore.kernel.org/lkml/878qb89g7b.ffs@tglx/

There are some loose ends in my patchset so I'm sending as a RFC to ask
some questions:

 - I haven't managed to expose the assembly labels correctly, the linker can't
 find it and the compilation fails, more info in patch 1/2
 - If the process is interrupted between the labels, we need to check the
 conditional flags and clear the op_pending address from the register. Using
 objdump I see that op_pending addr is being stored at x2, but I suspect that
 this isn't stable, so I need to figure out how to make sure that the address
 will always be stored in the same register.
 - So far I have implemented only the LL/SC version to make review easier, but I
 can do the LSE version as well.

This patchset works fine with the tests proposed at
https://lore.kernel.org/lkml/20260330120118.012924430@xxxxxxxxxx/ (but of course
without the labels the complete mechanism doesn't work properly).

---
André Almeida (2):
      arm64: vdso: Prepare for robust futex unlock support
      arm64: vdso: Implement __vdso_futex_robust_try_unlock()

 arch/arm64/Kconfig                                 |  1 +
 arch/arm64/include/asm/futex_robust.h              | 35 +++++++++++++
 arch/arm64/include/asm/vdso.h                      |  4 ++
 arch/arm64/kernel/vdso.c                           | 29 +++++++++++
 arch/arm64/kernel/vdso/Makefile                    |  9 +++-
 arch/arm64/kernel/vdso/vdso.lds.S                  |  4 ++
 .../kernel/vdso/vfutex_robust_list_try_unlock.c    | 59 ++++++++++++++++++++++
 7 files changed, 140 insertions(+), 1 deletion(-)
---
base-commit: 0e8896e9899b607bb168c1cce340596b8c2e3e2b
change-id: 20260416-tonyk-robust_arm-54ff77d2c4e4

Best regards,
--  
André Almeida <andrealmeid@xxxxxxxxxx>