From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id B60BD222565 for ; Wed, 11 Feb 2026 13:23:16 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770816196; cv=none; b=mFMnr5axX27GQN3IgDE6JjEdv1SZyTnSXOZ1HuGjxnuj/Y/j2e0OXH/G/eChcIKrsJWww+G7g6gxcH9EFw1iLxCCGgpN69VrDzhNq1uWMCWEcXxnj+rA/mB3zcyvmxxLp5tkLxv9/051Lu74Sk6lLnWg5xMqzeQKG7WqVZPkjgQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770816196; c=relaxed/simple; bh=JcQ+CTEL/lHWNFuYEMN/9SGhwKOoHjOvUBDU6QmKVDU=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=LKKPfWmozPwNojoc6+0iuXZVDcatNxVW5R1PycMNQpU6ifyOewdbVAFRfQm2BFp04ZWXH9Ur1n3Da/3oEFq9AAI1EUl1NsQRQ+/LDq0bXlmO3QPM7nWQll+Am2S8m+wOTnG0eyowXDCHWxfBtvmi94fONX+nA1/XQBr8xlOl3oc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=ERu8mOOS; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="ERu8mOOS" Received: by smtp.kernel.org (Postfix) with ESMTPSA id AFBCFC4CEF7; Wed, 11 Feb 2026 13:23:15 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1770816196; bh=JcQ+CTEL/lHWNFuYEMN/9SGhwKOoHjOvUBDU6QmKVDU=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=ERu8mOOSqrzNYSaiaD4GvnZaaNtHi68Zn8LUz0taczUebqfTT+27RdUMVAnKpv9Ue GCBmgQdnxJ8tgfHrTbL7Txfkvi5249aNhxf77JZmoUQwRLPUixt4bhZgQdmzHghPwu UdsmTbeToQBQF46+G52fmI60hzl0hPGm+EvVLKpylHrFoVqNvBLvDUfZ3HjrvPm9rT KSze6JpMzhORMvvPB9CBMBOsrTJ07cFKXFYOK4eaKO5Gi73wzx/XYd+o2B8D47Vs1U 4XuEQICXwKG6XD1uyeXiZZQLgtc2B6j2xUAUEoh5ISa32ZQwr/EFHpo5arbq02BwTp FGHnxeWZwsmhQ== Date: Wed, 11 Feb 2026 14:23:13 +0100 From: Alejandro Colomar To: =?utf-8?B?0L3QsNCx?= Cc: linux-man@vger.kernel.org Subject: Re: [PATCH v4] futex_waitv.2: new page Message-ID: References: Precedence: bulk X-Mailing-List: linux-man@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha512; protocol="application/pgp-signature"; boundary="lwdsusy5xlmxar4g" Content-Disposition: inline In-Reply-To: --lwdsusy5xlmxar4g Content-Type: text/plain; protected-headers=v1; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: quoted-printable From: Alejandro Colomar To: =?utf-8?B?0L3QsNCx?= Cc: linux-man@vger.kernel.org Subject: Re: [PATCH v4] futex_waitv.2: new page Message-ID: References: MIME-Version: 1.0 In-Reply-To: Hi =D0=BD=D0=B0=D0=B1, On 2026-02-11T05:00:26+0100, =D0=BD=D0=B0=D0=B1 wrote: > Signed-off-by: Ahelenia Ziemia=C5=84ska > --- > man/man2/futex_waitv.2 | 426 +++++++++++++++++++++++++++++++++++++++++ > man/man7/futex.7 | 9 +- > 2 files changed, 433 insertions(+), 2 deletions(-) > create mode 100644 man/man2/futex_waitv.2 >=20 > diff --git u/man/man2/futex_waitv.2 p/man/man2/futex_waitv.2 > new file mode 100644 > index 000000000..b05eb08ef > --- /dev/null > +++ p/man/man2/futex_waitv.2 > @@ -0,0 +1,426 @@ > +.\" Copyright, the authors of the Linux man-pages project > +.\" > +.\" SPDX-License-Identifier: MIT > +.\" > +.TH futex_waitv 2 (date) "Linux man-pages (unreleased)" > +.SH NAME > +futex_waitv \- wait for FUTEX_WAKE operation on multiple futexes > +.SH LIBRARY > +Standard C library > +.RI ( libc ,\~ \-lc ) > +.SH SYNOPSIS > +.nf > +.BR "#include " " /* Definition of " "struct futex_waitv= " " */" The comment should mention the FUTEX_ constants, and not the structure (which should be documented next to the structure). > +.BR "#include " " /* Definition of " SYS_* " constants *= /" > +.B #include > +.B #include > +.P > +.BR "long syscall(" "unsigned int nr_futexes;" Let's call this just 'n'. I have been wanting to globally trim parameter names that are unnecessarily verbose. Let's not add more. > +.BI " SYS_futex_waitv, struct futex_waitv " waiters [ nr_fut= exes ], > +.BI " unsigned int " nr_futexes ", unsigned int " flags , > +.BI " const struct timespec *_Nullable " timeout ", clockid_= t " clockid ");" > +.fi > +.P > +.EX Please include a header for this structure (consider as if the other headers were not included). > +struct futex_waitv { > + u64 val; /* Expected value at \f[I]uaddr\f[] */ > + u64 uaddr; /* User address to wait on */ > + u32 flags; /* Flags for this waiter */ > + u32 __reserved; /* Align to u64 */ > +}; > +.EE > +.SH DESCRIPTION > +.\" This name is used internally in the kernel > +Implements the FUTEX_WAIT_MULTIPLE operation, > +analogous to a synchronous atomic parallel > +.BR FUTEX_WAIT (2const) > +or > +.B FUTEX_WAIT_PRIVATE > +on up to > +.B FUTEX_WAITV_MAX > +futex words. > +For an overview of futexes, see > +.BR futex (7); > +for a description of the general interface, see > +.BR futex (2); > +for general minutiae of futex waiting, see the page above. > +.P > +This operation tests that the values at the > +futex words pointed to by the addresses > +.IR waiters []. uaddr > +still contain respective expected values > +.IR waiters []. val , > +and if so, sleeps waiting for a > +.BR FUTEX_WAKE (2const) > +operation on any of the futex words, > +and returns the index of > +.I a > +waiter whose futex was woken. > +.P > +If the thread starts to sleep, > +it is considered a waiter on all given futex words. > +If any of the futex values do not match their respective > +.IR waiters []. val , > +the call fails immediately with the error > +.BR EAGAIN . > +.P > +If > +.I timeout > +is not NULL, > +.I *timeout > +specifies a deadline measured against clock > +.IR clockid . > +This interval will be rounded up to the system clock granularity, > +and is guaranteed not to expire early. > +If > +.I timeout > +is NULL, the call blocks indefinitely. > +.P > +Futex words to monitor are given by > +.IR "struct futex_waitv" , > +whose fields are analogous to > +.BR FUTEX_WAIT (2const) > +parameters, except > +.I __reserved Please use .__reserved syntax for struct members (also with other members). > +must be 0 > +and > +.I flags > +must contain one of > +.BR FUTEX2_SIZE_* > +ORed with some of the flags below. I expect the flags should start immediately after 'below'. > +.P > +C programs should assign to > +.I uaddr > +by casting a pointer to > +.B uintptr_t > +to ensure the top bits are cleared on 32-bit systems. I'm not sure the paragraph above is correct. What code are you worried about exactly? Could you show an example? > +.TP > +.BR FUTEX2_SIZE_U32 > +.I val > +and > +.I *uaddr > +are 32-bit unsigned integers. > +.TP > +.B FUTEX2_NUMA > +The futex word is followed by another word of the same size > +.RI ( uaddr > +points to > +.IR uint N _t[2] > +rather than > +.IR uint N _t . > +The word is given by > +.IR uaddr[1] ), > +which can be either > +.B FUTEX_NO_NODE > +(all bits set) > +or a NUMA node number. > +.IP > +If the NUMA word is > +.BR FUTEX_NO_NODE , > +the node number of the processor the syscall executes on is written to i= t. > +(Except in an > +.B EINVAL > +or > +.B EFAULT > +condition, this happens to all waiters whose > +.I flags > +have > +.B FUTEX2_NUMA > +set.) > +.IP > +Futexes are placed on the NUMA node given by the NUMA word. > +Futexes without this flag are placed on a random node. > +.\" commit cec199c5e39bde7191a08087cc3d002ccfab31ff > +.\" Author: Peter Zijlstra > +.\" Date: Wed Apr 16 18:29:16 2025 +0200 > +.\" > +.\" futex: Implement FUTEX2_NUMA > +.\" > +.\" FUTEX2_MPOL is not documented or used anywhere; > +.\" it's unclear to me what it does > +.\" (defined in commit c042c505210dc3453f378df432c10fff3d471bc5 > +.\" "futex: Implement FUTEX2_MPOL") > +.TP > +.B FUTEX2_PRIVATE > +By default, the futex is shared > +.RB "(like " FUTEX_WAIT (2const)), > +and can be accessed by multiple processes; > +this flag waits on a private futex word, > +where all users must use the same virtual memory map > +(like > +.BR FUTEX_WAIT_PRIVATE ; > +this most often means they are part of the same process). > +Private futexes are faster than shared ones. > +.\" > +.\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" > +.\" > +.SH RETURN VALUE > +Returns an index to an arbitrary entry in > +.I waiters > +corresponding to some woken-up futex. > +This implies no information about other waiters. > +.P > +On error, > +\-1 is returned, > +and > +.I errno > +is set to indicate the error. > +.\" > +.\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" > +.\" > +.SH ERRORS > +.TP > +.B EFAULT > +.I waiters > +points outside the accessible address space. > +.TP > +.B EFAULT > +.I timeout > +was not NULL and did not point to a valid user-space address. > +.TP > +.B EFAULT > +Any > +.IR waiters []. uaddr > +field is not a valid user-space address. Please homogenize the wording of this and the first EFAULT. (I don't have a preference for which language is best). > +.TP > +.B EINVAL > +Any > +.IR waiters []. uaddr > +field does not point to a valid object\[em]that is, > +the address is not aligned appropriately for the specified > +.BR FUTEX2_SIZE_* . > +.TP > +.B EINVAL > +.I flags > +was not 0. > +.TP > +.B EINVAL > +.I nr_futexes > +was not in > +[1, > +.B FUTEX_WAITV_MAX > +(128)]. > +.TP > +.B EINVAL > +.I timeout > +was not NULL and > +.I clockid > +was not a valid clock > +.RB ( CLOCK_MONOTONIC > +or > +.BR CLOCK_REALTIME ). > +.TP > +.B EINVAL > +.I *timeout > +is denormal (before epoch or > +.I tv_nsec > +more than 999'999'999). ' should actually be \[aq]. > +.TP > +.B EINVAL > +Any > +.IR waiters []. flags > +field contains an unknown flag. > +.TP > +.B EINVAL > +Any > +.IR waiters []. flags > +field is missing a > +.B FUTEX2_SIZE_* > +flag or has a size flag different than > +.BR FUTEX2_SIZE_U32 > +set. > +.TP > +.B EINVAL > +Any > +.IR waiters []. __reserved > +field is not 0. > +.TP > +.B EINVAL > +Any > +.IR waiters []. value > +field has more bits set than permitted than the size flags. > +.TP > +.B EINVAL > +.B FUTEX2_NUMA > +was set in > +.IR waiters []. flags , > +and the NUMA word > +(which is the same size as the futex word) > +is too small to contain the index of the biggest NUMA domain > +(for example, > +.B FUTEX2_SIZE_U8 > +and there are more than 255 NUMA domains). > +.TP > +.B EINVAL > +.B FUTEX2_NUMA > +was set in > +.IR waiters []. flags , > +and the NUMA word is larger than the maximum possible NUMA node and not > +.BR FUTEX_NO_NODE . > +.TP > +.B ETIMEDOUT > +.I timeout > +was not NULL and no futex was woken before the timeout elapsed. > +.TP > +.BR EAGAIN " or " EWOULDBLOCK > +The value pointed to by > +.I uaddr > +was not equal to the expected value > +.I val > +at the time of the call. > +.TP > +.B EINTR > +The > +operation was interrupted by a signal (see > +.BR signal (7)). > +.\" > +.\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" > +.\" > +.SH STANDARDS > +Linux. > +.SH NOTES > +.BR FUTEX2_SIZE_U8 , > +.BR FUTEX2_SIZE_U16 , > +and > +.B FUTEX2_SIZE_U64 > +where > +.I val > +and > +.I *uaddr > +are 8, 16, or 64 bits are defined, but not implemented > +.RB ( EINVAL ). > +.SH HISTORY > +.\" commit bf69bad38cf63d980e8a603f8d1bd1f85b5ed3d9 > +.\" Author: Andr=C3=A9 Almeida > +.\" Date: Thu Sep 23 14:11:05 2021 -0300 > +.\" > +.\" futex: Implement sys_futex_waitv() > +Linux 5.16. > +.SH EXAMPLES > +The program below executes a linear-time operation on 10 threads, > +displaying the results in real time, > +waiting at most 1 second for each new result. > +The first 3 threads operate on the same data (complete in the same time). > +.B !\& > +indicates the futex that woke up each > +.BR futex_waitv (). > +.in +4 > +.EX > +.RB $\~ ./futex_waitv > +153 153 153 237 100 245 177 127 215 61 > + 122! > + 200! > + 254! > +306 306! > + 306! > + 354! > + 430! > + 474! > + 490! > +Connection timed out > +.EE > +.P > +.\" SRC BEGIN (futex_waitv.c) > +.EX > +#include > +#include > +#include > +#include > +#include > +#include > +#include > +#include > +#include > +#include > +#include > +#include > +\& > +static inline long > +my_futex_wait_private(_Atomic uint32_t *uaddr, uint32_t val, Should this be atomic? If so, should we document futex(2) as taking a pointer to atomic element? > + const struct timespec *_Nullable timeout) The two spaces are for variables, but not for function parameters. Also, my bad: this shouldn't have _Nullable. We don't use it in actual code (because Clang's _Nullable is really broken, and only serves documentation purposes). Actually, Clang is still sub-par in many ways; it's a C++ compiler, after all, where the C part is just a side-effect. > +{ > + return syscall(SYS_futex, uaddr, FUTEX_WAKE_PRIVATE, val, timeout); > +} > +\& > +static inline long > +my_futex_waitv(struct futex_waitv *waiters, unsigned int nr_futexes, > + unsigned int flags, const struct timespec *_Nullable ti= meout, > + clockid_t clockid) > +{ > + return syscall(SYS_futex_waitv, waiters, nr_futexes, flags, timeout, cl= ockid); > +} > +\& > +void * > +worker(void *arg) > +{ > + _Atomic uint32_t *futex =3D arg; > +\& > + usleep(*futex * 10000); > + *futex *=3D 2; > + my_futex_wait_private(futex, 1, NULL); > + return NULL; > +} > +\& > +int > +main(void) > +{ > + _Atomic uint32_t futexes[10]; > + uint8_t init[countof(futexes)]; > + struct futex_waitv waiters[countof(futexes)] =3D {}; > + int i; > +\& > + getentropy(init, sizeof(init)); > + init[0] =3D init[1] =3D init[2]; > + for (i =3D 0; i < countof(futexes); ++i) { > + printf("%" PRIu8 "\[rs]t", init[i]); > + atomic_init(&futexes[i], init[i]); > + pthread_create(&(pthread_t){}, NULL, worker, &futexes[i]); > + } > + putchar('\[rs]n'); > +\& > + for (i =3D 0; i < countof(futexes); ++i) { > + waiters[i].val =3D futexes[i]; > + waiters[i].uaddr =3D (uintptr_t)&futexes[i]; > + waiters[i].flags =3D FUTEX2_SIZE_U32 | FUTEX2_PRIVATE; > + } > + for (;;) { > + struct timespec timeout; > + int woke; > +\& > + clock_gettime(CLOCK_MONOTONIC, &timeout); > + timeout.tv_sec +=3D 1; > +\& > + woke =3D my_futex_waitv(waiters, countof(futexes), 0, &timeout, CLOCK_= MONOTONIC); > + if (woke =3D=3D -1 && (errno !=3D EAGAIN && errno !=3D EWOULDBLOCK)) > + break; > +\& > + for (i =3D 0; i < countof(futexes); ++i) { > + if (futexes[i] !=3D waiters[i].val) > + printf("%" PRIu32 "%s", futexes[i], i =3D=3D woke ? "!" : ""); > + putchar('\[rs]t'); > + } > + putchar('\[rs]n'); > +\& > + for (i =3D 0; i < countof(futexes); ++i) > + waiters[i].val =3D futexes[i]; > + } > + fprintf(stderr, "%s\[rs]n", strerror(errno)); > +} > +.EE > +.\" SRC END > +.SH SEE ALSO > +.ad l What's the effect of this ad l, and why do we want it? > +.BR futex (2), > +.BR FUTEX_WAIT (2const), > +.BR FUTEX_WAKE (2const), > +.BR futex (7) > +.P > +The following kernel source files: > +.IP \[bu] > +.I Documentation/userspace-api/futex2.rst I'd remove the below, and only point to the rst. Have a lovely day! Alex > +.IP \[bu] > +.I kernel/futex/syscall.c > +.IP \[bu] > +.I kernel/futex/waitwake.c > +.IP \[bu] > +.I kernel/futex/futex.h > diff --git u/man/man7/futex.7 p/man/man7/futex.7 > index 51c5d5d9b..d271144ff 100644 > --- u/man/man7/futex.7 > +++ p/man/man7/futex.7 > @@ -45,7 +45,9 @@ .SS Semantics > Any futex operation starts in user space, > but it may be necessary to communicate with the kernel using the > .BR futex (2) > -system call. > +or > +.BR futex_waitv (2) > +system calls. > .P > To "up" a futex, execute the proper assembler instructions that > will cause the host CPU to atomically increment the integer. > @@ -72,7 +74,9 @@ .SS Semantics > .P > The > .BR futex (2) > -system call can optionally be passed a timeout specifying how long > +and > +.BR futex_waitv (2) > +system calls can optionally be passed a timeout specifying how long > the kernel should > wait for the futex to be upped. > In this case, semantics are more complex and the programmer is referred > @@ -107,6 +111,7 @@ .SH NOTES > .SH SEE ALSO > .BR clone (2), > .BR futex (2), > +.BR futex_waitv (2), > .BR get_robust_list (2), > .BR set_robust_list (2), > .BR set_tid_address (2), > --=20 > 2.39.5 --=20 --lwdsusy5xlmxar4g Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQIzBAABCgAdFiEES7Jt9u9GbmlWADAi64mZXMKQwqkFAmmMgrsACgkQ64mZXMKQ wqmx3g/+OTpXig9Przatiy/bIJFqisJbF8IX2XTeXR2Ei8BsM2BS6Cfcf/tdrhft BhtMHSBWPolRWuIoASLaj2EvLr48IiFMBHEMG6xdPQtPpt2pneLbw4pdWn+jznkF 9INqzmaAJC6MR3BfVy/gmKweqCvBq2MuHdv1C2kHg63QNegHbPz4Tky0vSzEZ47F vVYdQE8hMiXSB/8Zxo72O9JeBWO6xI6ohZENpAWA/4+lWzzkwjYXfWA/vYgTDjN1 q3rRXkB54DB7BLpOxealrHgfdjRZgmDOwRw65fjpWuFCrE2VL7mIirL0qXDLPB4S 1XTrjGo4gn+AcWoR1nRXn8ze5F7kWjyCT+zNWzEf5bFH7szxAHCUSF+55gTSaPT/ DLxEl8HP3N/TnBS5OyRaJF7nPhtBw016QvCWjEQxDybp4/pLl7nC+MeogjoK2JBP 1GvM6EgYcaKq3NHTADQD28A8j/Yx1iPsAQ7JM/mEIIF1nlc+X6pIXq9985BUKnas 0wXnQtoACALl/n6R6XmqEG9sXVTr0lzc1Sbbp/qfcsB4pI7z1yp0i3jGGh/lw4hV 2BDCQGfVVGp3IGG3aeMHwb0C56+0fdd2xhyh7TUsk9dU9VWgYyNvZ+5Ij7T3AGK8 dKoUchtztgNW9ggWTJAFNX1iL42RLhwSG1lFNvN37KFN/2Qyq8U= =ophU -----END PGP SIGNATURE----- --lwdsusy5xlmxar4g--