Re: [RFC PATCH 0/2] Linux: Add futex_waitv syscall wrapper

Hi Adhemerval,

Em 20/08/2026 14:41, Adhemerval Zanella Netto escreveu:
> 
> 
> On 18/08/26 14:57, Adhemerval Zanella Netto wrote:
>>
>>
>> On 14/08/26 15:36, André Almeida wrote:
>>> Hi folks,
>>>
>>> Based on a talk on last LPC by Carlos O'Donell[1], I'm sending this patch as a
>>> starting point to add the futex_waitv() syscall wrapper.
>>>
>>> I tried to base myself on the work done for others wrappers, but I see that
>>> there are missing details in my implementation, so I'm seeking guidance on how
>>> to approach this.
>>>
>>
>>
>> I will try to create POC so we can check if it makes sense.
> I create a POC on personal branch for a possible futex interface [1]. Some design
> notes:
> 
> * Only the well-defined subset of futex(2) is exposed: wait, timed wait, wake,
>    value-checked requeue, and futex_waitv. The racy unchecked FUTEX_REQUEUE, the
>    removed FUTEX_FD, the error-prone FUTEX_WAKE_OP, bitset matching, and the PI
>    operations are deliberately not exposed.
> 
> * Survey of real futex users (libgomp, libitm, libstdc++ C++11/C++20 tiers, libgo,
>    libc++, OpenMP runtime, scudo, sanitizers) shows every usage in the wild is
>    expressible with this set. The only caller-side adaptation anywhere is
>    relative→absolute timeout conversion (libgo, libc++).
> 
> * For the futex_* interfaces I also added a 32-bit interface (which I suggested
>    against). It seems that libstc++ does not use _TIME_BITS and issues the syscall
>    directly (and it always use the 32 bit time_t futex one, which is different
>    issue). It makes a possible integration easier and it also seems expected
>    to follow suit other syscall that still have 32-bit time_t support.
> 
> * futex_waitv is only provided for 64-bit time_t.
> 
> * All of them uses the default -1/errno for error reporting, instead of the
>    pthread like way I suggested. With the only exception of libgo from gcc, all
>    other futexes usage are through 'syscall' and they handle errors through the
>    usual errno mechanism.
> 
> * Flags follow the kernel convention: zero means process-shared, FUTEX_PRIVATE_FLAG
>    selects the private fast path. So the same value works across all five functions,
>    including futex_waitv's per-waiter FUTEX2_PRIVATE.
> 
> * futex_timedwait takes an absolute timeout with an explicit clockid, implemented
>    with FUTEX_WAIT_BITSET/FUTEX_BITSET_MATCH_ANY to avoid the relative-only
>    CLOCK_MONOTONIC quirk of FUTEX_WAIT. Pre-epoch timeouts are reported as ETIMEDOUT
>    instead of the kernel's EINVAL (as we do for other syscalls).
> 
> * The exported FUTEX2_* constants cover only what the syscall accepts today
>    (FUTEX2_SIZE_U32, FUTEX2_PRIVATE, FUTEX2_NUMA, FUTEX2_MPOL, FUTEX_NO_NODE). The
>    unimplemented sizes are omitted.
> 
> [1] https://sourceware.org/git?p=glibc.git;a=shortlog;h=refs/heads/azanella/futex-wrappers
Thank you! I had a look and I believe this is the right direction for 
the futex wrappers.

Please let me know how can I cooperate with this effort, if there's any 
patch left to be written, if you need tests or so.