linux-kernel - Re: [[PATCH v3] 2/4] man/man2/prctl.2, PR_FUTEX_HASH_SET_SLOTS.2const: Document PR_FUTEX_HASH_SET

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Message-ID: <b6sst5trs54mco6ah4wnk3ntnj6wi7diskothhell7yc4ppph4@5pugtx4mzljx>
Date: Fri, 30 May 2025 11:59:31 +0200
From: Alejandro Colomar <alx@...nel.org>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: linux-kernel@...r.kernel.org, linux-man@...r.kernel.org, 
	André Almeida <andrealmeid@...lia.com>, Darren Hart <dvhart@...radead.org>, 
	Davidlohr Bueso <dave@...olabs.net>, Ingo Molnar <mingo@...hat.com>, 
	Juri Lelli <juri.lelli@...hat.com>, Peter Zijlstra <peterz@...radead.org>, 
	Thomas Gleixner <tglx@...utronix.de>, Valentin Schneider <vschneid@...hat.com>, 
	Waiman Long <longman@...hat.com>
Subject: Re: [[PATCH v3] 2/4] man/man2/prctl.2,
 PR_FUTEX_HASH_SET_SLOTS.2const: Document PR_FUTEX_HASH_SET_SLOTS

Hi Sebastian,

On Mon, May 26, 2025 at 05:55:21PM +0200, Sebastian Andrzej Siewior wrote:
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> ---
>  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

We format constants in bold.  See man-pages(7):

   Formatting conventions (general)
[...]
     Special macros, which are usually in uppercase, are in bold (e.g.,
     MAXINT).  Exception: don’t boldface NULL.
[...]
     Expressions, if not written on a separate indented line, should be
     specified in italics.  Again, the use of nonbreaking spaces may be
     appropriate if the expression is inlined with normal text.


> +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

I think the second RE does not match any previous RS.

> +.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

Have a lovely day!
Alex

-- 
<https://www.alejandro-colomar.es/>

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