[PATCH v4 2/4] man/man2/prctl.2, PR_FUTEX_HASH_SET_SLOTS.2const: Document PR_FUTEX_HASH_SET_SLOTS

Sebastian Andrzej Siewior
2025-06-02T14:01:10Z

man/man2const/PR_FUTEX_HASH_SET_SLOTS.2const | 83 ++++++++++++++++++++
1 file changed, 83 insertions(+)
create mode 100644 man/man2const/PR_FUTEX_HASH_SET_SLOTS.2const

diff --git a/man/man2const/PR_FUTEX_HASH_SET_SLOTS.2const b/man/man2const/PR_FUTEX_HASH_SET_SLOTS.2const
new file mode 100644
index 0000000000000..1f08d1bb30485
--- /dev/null
+++ b/man/man2const/PR_FUTEX_HASH_SET_SLOTS.2const
@@ -0,0 +1,83 @@
+.\" Copyright, the authors of the Linux man-pages project
+.\"
+.\" SPDX-License-Identifier: Linux-man-pages-copyleft
+.\"
+.TH PR_FUTEX_HASH_SET_SLOTS 2const (date) "Linux man-pages (unreleased)"
+.SH NAME
+PR_FUTEX_HASH_SET_SLOTS
+\-
+set the size of the private hash
+.SH LIBRARY
+Standard C library
+.RI ( libc ,\~ \-lc )
+.SH SYNOPSIS
+.nf
+.BR "#include <linux/prctl.h>" " /* Definition of " PR_* " constants */"
+.B #include <sys/prctl.h>
+.P
+.B int prctl(PR_FUTEX_HASH, PR_FUTEX_HASH_SET_SLOTS,
+.BI " unsigned long " hash_size ", unsigned long " hash_flags ");
+.fi
+.SH DESCRIPTION
+Set the number of slots to use for the private hash.
+.TP
+.I hash_size
+Specifies the size of private hash to allocate.
+.RS
+.TP
+.I 0
+Use the global hash.
+This is the behaviour used before 6.16.
+The operation implies the
+.I FH_FLAG_IMMUTABLE
+flag.
+.TP
+.I >0
+Specifies the number of slots to allocate.
+The value must be power of two and the lowest possible value is 2.
+The upper limit depends on the available memory in the system.
+Each slot requires 64bytes of memory.
+Kernels compiled with
+.I CONFIG_PROVE_LOCKING
+will consume more than that.
+.RE
+.TP
+.I hash_flags
+.RS
+The following flags can be specified:
+.TP
+.I FH_FLAG_IMMUTABLE
+The private hash can no longer be changed.
+By using an immutable private hash
+the kernel can avoid some accounting for the data structure.
+This accounting is visible in benchmarks if many
+.BR futex (2)
+operations are invoked in parallel on different CPUs.
+.RE
+.RE
+.SH RETURN VALUE
+On success,
+0 is returned.
+On error, \-1 is returned, and
+.I errno
+is set to indicate the error.
+.SH ERRORS
+.TP
+.B EINVAL
+One of the supplied argument is invalid.
+.TP
+.B ENOMEM
+Failed to allocate memory.
+.TP
+.B EBUSY
+An immutable hash is already in use
+and can not be changed.
+.SH STANDARDS
+Linux.
+.SH HISTORY
+Linux 6.16.
+.SH SEE ALSO
+.BR prctl (2),
+.BR PR_FUTEX_HASH (2const),
+.BR PR_FUTEX_HASH_GET_IMMUTABLE (2const),
+.BR PR_FUTEX_HASH_GET_SLOTS (2const)
--
2.49.0