On 2026-02-11T15:15:35+0100, наб wrote: > Signed-off-by: Ahelenia Ziemiańska > --- > Same as v5 but FUTEX_WAKE_PRIVATE is no longer given a timeout argument > it doesn't use. I'd appreciate range-diffs to be able to understand what actually changes in patch revisions. See . Cheers, Alex > > man/man2/futex_waitv.2 | 418 +++++++++++++++++++++++++++++++++++++++++ > man/man7/futex.7 | 9 +- > 2 files changed, 425 insertions(+), 2 deletions(-) > create mode 100644 man/man2/futex_waitv.2 > > diff --git u/man/man2/futex_waitv.2 p/man/man2/futex_waitv.2 > new file mode 100644 > index 000000000..bc6f7528c > --- /dev/null > +++ p/man/man2/futex_waitv.2 > @@ -0,0 +1,418 @@ > +.\" 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 " FUTEX* " constants */" > +.BR "#include " " /* Definition of " SYS_* " constants */" > +.B #include > +.B #include > +.P > +.BR "long syscall(" "unsigned int n;" > +.BI " SYS_futex_waitv, struct futex_waitv " waiters [ n ], > +.BI " unsigned int " n ", unsigned int " flags , > +.BI " const struct timespec *_Nullable " timeout ", clockid_t " clockid ");" > +.fi > +.P > +.EX > +.B "#include " > +.P > +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 > +must be 0 > +and > +.I .flags > +must contain one of > +.BR FUTEX2_SIZE_* > +ORed with some of the flags below. > +.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 it. > +(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. > +.P > +C programs should assign to > +.I .uaddr > +by casting a pointer to > +.BR uintptr_t . > +.\" > +.\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" > +.\" > +.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 > +is not NULL and points outside the accessible address space. > +.TP > +.B EFAULT > +Any > +.IR waiters []. uaddr > +field points outside the accessible address space. > +.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 n > +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\[aq]999\[aq]999). > +.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: André 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) > +{ > + return syscall(SYS_futex, uaddr, FUTEX_WAKE_PRIVATE, val); > +} > +\& > +static inline long > +my_futex_waitv(struct futex_waitv *waiters, unsigned int n, > + unsigned int flags, const struct timespec *timeout, > + clockid_t clockid) > +{ > + return syscall(SYS_futex_waitv, waiters, n, flags, timeout, clockid); > +} > +\& > +void * > +worker(void *arg) > +{ > + _Atomic uint32_t *futex = arg; > +\& > + usleep(*futex * 10000); > + *futex *= 2; > + my_futex_wait_private(futex, 1); > + return NULL; > +} > +\& > +int > +main(void) > +{ > + _Atomic uint32_t futexes[10]; > + uint8_t init[countof(futexes)]; > + struct futex_waitv waiters[countof(futexes)] = {}; > + int i; > +\& > + getentropy(init, sizeof(init)); > + init[0] = init[1] = init[2]; > + for (i = 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 = 0; i < countof(futexes); ++i) { > + waiters[i].val = futexes[i]; > + waiters[i].uaddr = (uintptr_t)&futexes[i]; > + waiters[i].flags = FUTEX2_SIZE_U32 | FUTEX2_PRIVATE; > + } > + for (;;) { > + struct timespec timeout; > + int woke; > +\& > + clock_gettime(CLOCK_MONOTONIC, &timeout); > + timeout.tv_sec += 1; > +\& > + woke = my_futex_waitv(waiters, countof(futexes), 0, &timeout, CLOCK_MONOTONIC); > + if (woke == -1 && (errno != EAGAIN && errno != EWOULDBLOCK)) > + break; > +\& > + for (i = 0; i < countof(futexes); ++i) { > + if (futexes[i] != waiters[i].val) > + printf("%" PRIu32 "%s", futexes[i], i == woke ? "!" : ""); > + putchar('\[rs]t'); > + } > + putchar('\[rs]n'); > +\& > + for (i = 0; i < countof(futexes); ++i) > + waiters[i].val = futexes[i]; > + } > + fprintf(stderr, "%s\[rs]n", strerror(errno)); > +} > +.EE > +.\" SRC END > +.SH SEE ALSO > +.BR futex (2), > +.BR FUTEX_WAIT (2const), > +.BR FUTEX_WAKE (2const), > +.BR futex (7) > +.P > +Kernel source file > +.I Documentation/userspace-api/futex2.rst > 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), > -- > 2.39.5 --