Re: [patch v2 11/11] x86/vdso: Implement __vdso_futex_robust_try_unlock()

[Uros Bizjak]

On Fri, Mar 20, 2026 at 12:25 AM Thomas Gleixner <tglx@xxxxxxxxxx> wrote:

>         mov             %esi,%eax       // Load TID into EAX
>         xor             %ecx,%ecx       // Set ECX to 0
>         lock cmpxchg    %ecx,(%rdi)     // Try the TID -> 0 transition
>   .Lstart:
>         jnz             .Lend
>         movq            %rcx,(%rdx)     // Clear list_op_pending
>   .Lend:
>         ret

[...]

> + * Assembly template for the try unlock functions. The basic functionality is:
> + *
> + *             mov             esi, %eax       Move the TID into EAX
> + *             xor             %ecx, %ecx      Clear ECX
> + *             lock_cmpxchgl   %ecx, (%rdi)    Attempt the TID -> 0 transition
> + * .Lcs_start:                                 Start of the critical section
> + *             jnz             .Lcs_end        If cmpxchl failed jump to the end
> + * .Lcs_success:                               Start of the success section
> + *             movq            %rcx, (%rdx)    Set the pending op pointer to 0
> + * .Lcs_end:                                   End of the critical section
> + *
> + * .Lcs_start and .Lcs_end establish the critical section range. .Lcs_success is
> + * technically not required, but there for illustration, debugging and testing.
> + *
> + * When CONFIG_COMPAT is enabled then the 64-bit VDSO provides two functions.
> + * One for the regular 64-bit sized pending operation pointer and one for a
> + * 32-bit sized pointer to support gaming emulators.
> + *
> + * The 32-bit VDSO provides only the one for 32-bit sized pointers.
> + */
> +#define __stringify_1(x...)    #x
> +#define __stringify(x...)      __stringify_1(x)
> +
> +#define LABEL(name, which)     __stringify(name##_futex_try_unlock_cs_##which:)
> +
> +#define JNZ_END(name)          "jnz " __stringify(name) "_futex_try_unlock_cs_end\n"
> +
> +#define CLEAR_POPQ             "movq   %[zero],  %a[pop]\n"
> +#define CLEAR_POPL             "movl   %k[zero], %a[pop]\n"
> +
> +#define futex_robust_try_unlock(name, clear_pop, __lock, __tid, __pop) \
> +({                                                                     \
> +       asm volatile (                                                  \
> +               "                                               \n"     \
> +               "       lock cmpxchgl   %k[zero], %a[lock]      \n"     \
> +               "                                               \n"     \
> +               LABEL(name, start)                                      \
> +               "                                               \n"     \
> +               JNZ_END(name)                                           \
> +               "                                               \n"     \
> +               LABEL(name, success)                                    \
> +               "                                               \n"     \
> +                       clear_pop                                       \
> +               "                                               \n"     \
> +               LABEL(name, end)                                        \
> +               : [tid]   "+&a" (__tid)                                 \
> +               : [lock]  "D"   (__lock),                               \
> +                 [pop]   "d"   (__pop),                                \
> +                 [zero]  "S"   (0UL)                                   \

[zero] represents an internal register, so the above constraint can be
"r" (*). If it remains a hard register constraint (%rsi) for some
reason then the above two comments should be updated to reflect the
new constraint.

With the constraint changed to "r":

Acked-by: Uros Bizjak <ubizjak@xxxxxxxxx> (for asm template)

(*) "r" allows the compiler some more freedom. The compiler tracks the
values in registers, so it can reuse zero from an unrelated register
without moving it to the %rsi and without clobbering the source
register. In non-trivial functions, there is a high chance that needed
value is already available in some register.

Uros.