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

From: André Almeida <[email-masked]>
Date: Mon, 1 Jun 2026 13:22:28 -0300

Em 29/05/2026 14:47, Mathieu Desnoyers escreveu:
> On 2026-05-29 12:33, 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
> 
> So it looks like stlxr can be successful, and the process is killed
> right here. This is not within the start/end critical section, so the
> fixup is missed ? Or am I missing something ?
> 

As me and Mathieu discussed on IRC, this should be the right thing to do 
here:

  - Move the critical section label one instruction above to include 
`cbnz` on it
  - Use the result register as the check for the store success, not the 
zero flag register.

So it will look like this:

	__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
	__vdso_futex_list64_try_unlock_cs_start:
		cbnz	w9, retry
		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 result register:

	return (regs->user_regs[9]) ? NULL :
		(void __user *) regs->user_regs.regs[2];

Thanks for the review!