Re: [PATCH 1/1] rseq.2: New man page for the rseq(2) API

[Alejandro Colomar]

Hi Mathieu and Branden,

On 2/14/23 20:54, Mathieu Desnoyers wrote:
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> ---
>  man2/rseq.2 | 465 ++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 465 insertions(+)
>  create mode 100644 man2/rseq.2
> 
> diff --git a/man2/rseq.2 b/man2/rseq.2
> new file mode 100644
> index 000000000..ad3215d1c
> --- /dev/null
> +++ b/man2/rseq.2
> @@ -0,0 +1,465 @@
> +.\" Copyright 2015-2023 Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> +.\"
> +.\" SPDX-License-Identifier: Linux-man-pages-copyleft
> +.\"
> +.TH rseq 2 (date) "Linux man-pages (unreleased)"
> +.SH NAME
> +rseq \- restartable sequences system call
> +.SH LIBRARY
> +Standard C library
> +.RI ( libc ", " \-lc )
> +.SH SYNOPSIS
> +.nf
> +.PP
> +.BR "#include <linux/rseq.h>" "       /* Definition of " RSEQ_* " constants */"
> +.BR "#include <sys/syscall.h>" "      /* Definition of " SYS_* " constants */"
> +.B #include <unistd.h>
> +.PP
> +.BI "int syscall(SYS_rseq, struct rseq *" rseq ", uint32_t " rseq_len ,
> +.BI "            int " flags ", uint32_t " sig );
> +.fi
> +.PP
> +.IR Note :
> +glibc provides no wrapper for
> +.BR rseq (),
> +necessitating the use of
> +.BR syscall (2).
> +.SH DESCRIPTION
> +The
> +.BR rseq ()
> +ABI accelerates specific user-space operations by registering a
> +per-thread data structure shared between kernel and user-space.

This last 'user-space' is not adjectivated, so it should go without
a hyphen, according to common English rules.

> +This data structure can be read from or written to by user-space to skip

same here

> +otherwise expensive system calls.
> +.PP
> +A restartable sequence is a sequence of instructions
> +guaranteed to be executed atomically with respect to
> +other threads and signal handlers on the current CPU.
> +If its execution does not complete atomically,
> +the kernel changes the execution flow by jumping to an abort handler
> +defined by user-space for that restartable sequence.

same here

> +.PP
> +Using restartable sequences requires to register a
> +.BR rseq ()
> +ABI per-thread data structure
> +.RB ( "struct rseq" )

We format types in italics, so this should be '.RI'.

> +through the
> +.BR rseq ()
> +system call.
> +Only one
> +.BR rseq ()
> +ABI can be registered per thread, so user-space libraries and
> +applications must follow a user-space ABI defining how to share this
> +resource.

Please use semantic newlines.  See man-pages(7):

   Use semantic newlines
       In  the source of a manual page, new sentences should be started on new
       lines, long sentences should be split into lines at clause breaks (com‐
       mas, semicolons, colons, and so on), and long clauses should  be  split
       at  phrase  boundaries.   This convention, sometimes known as "semantic
       newlines", makes it easier to see the effect of  patches,  which  often
       operate at the level of individual sentences, clauses, or phrases.

In the above lines, that would mean breaking after the comma, and not leaving resource in a line of its own.

> +The ABI defining how to share this resource between applications and
> +libraries is defined by the C library.
> +Allocation of the per-thread
> +.BR rseq ()
> +ABI and its registration to the kernel is handled by glibc since version
> +2.35.
> +.PP
> +The
> +.BR rseq ()
> +ABI per-thread data structure contains a
> +.I rseq_cs
> +field which points to the currently executing critical section.

currently-executing should probably use a hyphen
(if I understood the line correctly).
However, you might want to reorder the phrase to remove the need for that.

See an interesting discussion in the groff@ mailing list:
<https://lists.gnu.org/archive/html/groff/2022-10/msg00015.html>

> +For each thread, a single rseq critical section can run at any given
> +point.
> +Each critical section need to be implemented in assembly.

needs?

> +.PP
> +The
> +.BR rseq ()
> +ABI accelerates user-space operations on per-cpu data by defining a
> +shared data structure ABI between each user-space thread and the kernel.
> +.PP
> +It allows user-space to perform update operations on per-cpu data
> +without requiring heavy-weight atomic operations.
> +.PP
> +The term CPU used in this documentation refers to a hardware execution
> +context.
> +For instance, each CPU number returned by
> +.BR sched_getcpu ()
> +is a CPU.
> +The current CPU means to the CPU on which the registered thread is
> +running.
> +.PP
> +Restartable sequences are atomic with respect to preemption (making it
> +atomic with respect to other threads running on the same CPU),
> +as well as signal delivery (user-space execution contexts nested over
> +the same thread).
> +They either complete atomically with respect to preemption on the
> +current CPU and signal delivery, or they are aborted.
> +.PP
> +Restartable sequences are suited for update operations on per-cpu data.
> +.PP
> +Restartable sequences can be used on data structures shared between threads
> +within a process,
> +and on data structures shared between threads across different
> +processes.
> +.PP
> +Some examples of operations that can be accelerated or improved by this ABI:
> +.IP \(bu 3

Please use \[bu]

We changed that recently:
<https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/commit/?id=36f73ba37945c7ff4aa2d8383f831519a38e3f27>

> +Memory allocator per-cpu free-lists,
> +.IP \(bu 3
> +Querying the current CPU number,
> +.IP \(bu 3
> +Incrementing per-CPU counters,
> +.IP \(bu 3
> +Modifying data protected by per-CPU spinlocks,
> +.IP \(bu 3
> +Inserting/removing elements in per-CPU linked-lists,
> +.IP \(bu 3
> +Writing/reading per-CPU ring buffers content.
> +.IP \(bu 3
> +Accurately reading performance monitoring unit counters with respect to
> +thread migration.
> +.PP
> +Restartable sequences must not perform system calls.
> +Doing so may result in termination of the process by a segmentation
> +fault.
> +.PP
> +The
> +.I rseq
> +argument is a pointer to the thread-local
> +.B struct rseq
> +to be shared between kernel and user-space.
> +.PP
> +The structure
> +.B struct rseq
> +is an extensible structure.
> +Additional feature fields can be added in future kernel versions.
> +Its layout is as follows:
> +.TP
> +.B Structure alignment

Let's remove the bold here.  It's not necessary for marking a constant or something that needs bold.  And the indentation is already making it stand out, so bold is a bit too much aggressive to the reader.

> +This structure is aligned on either 32-byte boundary,
> +or on the alignment value returned by
> +.IR getauxval ()
> +invoked with
> +.B AT_RSEQ_ALIGN
> +if the structure size differs from 32 bytes.
> +.TP
> +.B Structure size
> +This structure size needs to be at least 32 bytes.
> +It can be either 32 bytes,
> +or it needs to be large enough to hold the result of
> +.IR getauxval ()
> +invoked with
> +.BR AT_RSEQ_FEATURE_SIZE .

Maybe it's simpler to use getauxval(AT_RSEQ_FEATURE_SIZE) as an expression?

If you decide to do so, it should go in italics (see man-pages(7)).

       Complete commands should, if long, be written as an  indented  line  on
       their own, with a blank line before and after the command, for example

           man 7 man-pages

       If the command is short, then it can be included inline in the text, in
       italic  format,  for example, man 7 man‐pages.  In this case, it may be
       worth using nonbreaking spaces (\[ti]) at suitable places in  the  com‐
       mand.  Command options should be written in italics (e.g., -l).

       Expressions,  if  not  written  on  a separate indented line, should be
       specified in italics.  Again, the use of nonbreaking spaces may be  ap‐
       propriate if the expression is inlined with normal text.


> +Its size is passed as parameter to the
> +.BR rseq ()
> +system call.
> +.in +4n
> +.IP
> +.EX
> +#include <linux/rseq.h>
> +
> +struct rseq {
> +    __u32 cpu_id_start;
> +    __u32 cpu_id;
> +    union {
> +        /* ... */
> +    } rseq_cs;
> +    __u32 flags;
> +    __u32 node_id;
> +    __u32 mm_cid;
> +} __attribute__((aligned(32)));
> +.EE
> +.in
> +.TP
> +.B Fields
> +.RS
> +.TP
> +.I cpu_id_start
> +Always-updated value of the CPU number on which the registered thread is
> +running.
> +Its value is guaranteed to always be a possible CPU number,
> +even when
> +.BR rseq ()
> +is not registered.
> +Its value should always be confirmed by reading the cpu_id field before

cpu_id should be formatted (.I).

> +user-space performs any side-effect
> +(e.g. storing to memory).
> +.IP
> +This field is always guaranteed to hold a valid CPU number in the range
> +[ 0 ..  nr_possible_cpus - 1 ].

Please use interval notation:
	[0, nr_possible_cpus)
or
	[0, nr_possible_cpus - 1]
whichever looks better to you.

We did some consistency fix recently:
<https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/commit/?id=147a60d792a5db8f3cb93ea16eefb73e16c1fb91>

Also, do we have a more standard way of saying nr_possible_cpus?
Should we say nproc?

> +It can therefore be loaded by user-space
> +and used as an offset in per-cpu data structures
> +without having to check whether its value is within the valid bounds
> +compared to the number of possible CPUs in the system.
> +.IP
> +Initialized by user-space to a possible CPU number (e.g., 0),
> +updated by the kernel for threads registered with
> +.BR rseq ().
> +.IP
> +For user-space applications executed on a kernel without
> +.BR rseq ()
> +support,
> +the cpu_id_start field stays initialized at 0,
> +which is indeed a valid CPU number.
> +It is therefore valid to use it as an offset in per-cpu data structures,
> +and only validate whether it's actually the current CPU number by
> +comparing it with the cpu_id field within the rseq critical section.
> +If the kernel does not provide
> +.BR rseq ()
> +support, that cpu_id field stays initialized at -1,
> +so the comparison always fails, as intended.
> +.IP
> +This field should only be read by the thread which registered this data
> +structure.
> +Aligned on 32-bit.
> +.IP
> +It is up to user space to implement a fall-back mechanism for scenarios where
> +.BR rseq ()
> +is not available.
> +.TP
> +.I cpu_id
> +Always-updated value of the CPU number on which the registered thread is
> +running.
> +Initialized by user-space to -1,
> +updated by the kernel for threads registered with
> +.BR rseq ().
> +.IP
> +This field should only be read by the thread which registered this data
> +structure.
> +Aligned on 32-bit.
> +.TP
> +.I rseq_cs
> +The rseq_cs field is a pointer to a
> +.BR "struct rseq_cs" .
> +Is is NULL when no rseq assembly block critical section is active for

"It is"

> +the registered thread.
> +Setting it to point to a critical section descriptor
> +.RB ( "struct rseq_cs")
> +marks the beginning of the critical section.
> +.IP
> +Initialized by user-space to NULL.
> +.IP
> +Updated by user-space, which sets the address of the currently
> +active rseq_cs at the beginning of assembly instruction sequence
> +block,
> +and set to NULL by the kernel when it restarts an assembly instruction
> +sequence block,
> +as well as when the kernel detects that it is preempting or delivering a
> +signal outside of the range targeted by the rseq_cs.
> +Also needs to be set to NULL by user-space before reclaiming memory that
> +contains the targeted
> +.BR "struct rseq_cs" .
> +.IP
> +Read and set by the kernel.
> +.IP
> +This field should only be updated by the thread which registered this
> +data structure.
> +Aligned on 64-bit.
> +.TP
> +.I flags
> +Flags indicating the restart behavior for the registered thread.
> +This is mainly used for debugging purposes.
> +Can be a combination of:
> +.RS
> +.TP
> +.B RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT
> +Inhibit instruction sequence block restart on preemption for this
> +thread.
> +This flag is deprecated since Linux 6.1.
> +.TP
> +.B RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL
> +Inhibit instruction sequence block restart on signal delivery for this
> +thread.
> +This flag is deprecated since Linux 6.1.
> +.TP
> +.B RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE
> +Inhibit instruction sequence block restart on migration for this thread.
> +This flag is deprecated since Linux 6.1.
> +.RE
> +.IP
> +Initialized by user-space, used by the kernel.
> +.TP
> +.I node_id
> +Always-updated value of the current NUMA node ID.
> +.IP
> +Initialized by user-space to 0.
> +.IP
> +Updated by the kernel.
> +Read by user-space with single-copy atomicity semantics.
> +This field should only be read by the thread which registered
> +this data structure.
> +Aligned on 32-bit.
> +.TP
> +.I mm_cid
> +Contains the current thread's concurrency ID
> +(allocated uniquely within a memory map).
> +.IP
> +Updated by the kernel.
> +Read by user-space with single-copy atomicity semantics.
> +This field should only be read by the thread which registered this data
> +structure.
> +Aligned on 32-bit.
> +.IP
> +This concurrency ID is within the possible cpus range,
> +and is temporarily (and uniquely) assigned while threads are actively
> +running within a memory map.
> +If a memory map has fewer threads than cores,
> +or is limited to run on few cores concurrently through sched affinity or
> +cgroup cpusets,
> +the concurrency IDs will be values close to 0,
> +thus allowing efficient use of user-space memory for per-cpu data
> +structures.
> +.RE
> +.PP
> +The layout of
> +.B struct rseq_cs
> +version 0 is as follows:
> +.TP
> +.B Structure alignment
> +This structure is aligned on 32-byte boundary.
> +.TP
> +.B Structure size
> +This structure has a fixed size of 32 bytes.
> +.in +4n
> +.IP
> +.EX
> +#include <linux/rseq.h>
> +
> +struct rseq_cs {
> +    __u32   version;
> +    __u32   flags;
> +    __u64   start_ip;
> +    __u64   post_commit_offset;
> +    __u64   abort_ip;
> +} __attribute__((aligned(32)));
> +.EE
> +.in
> +.TP
> +.B Fields

Branden, IIRC, this seems to be the reason why I didn't want .RS for indenting code examples.  It doesn't fit nicely right after TP.

Why is there a blank line?  I'm not even sure that's reasonable.  Is it a (minor) bug in man(7)?  (FWIW, mandoc(1) is consistent with groff(1).)

> +.RS
> +.TP
> +.I version
> +Version of this structure.
> +Should be initialized to 0.
> +.TP
> +.I flags
> +.RS
> +Flags indicating the restart behavior of this structure.
> +Can be a combination of:
> +.TP
> +.B RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT
> +Inhibit instruction sequence block restart on preemption for this
> +critical section.
> +This flag is deprecated since Linux 6.1.
> +.TP
> +.B RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL
> +Inhibit instruction sequence block restart on signal delivery for this
> +critical section.
> +This flag is deprecated since Linux 6.1.
> +.TP
> +.B RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE
> +Inhibit instruction sequence block restart on migration for this
> +critical section.
> +This flag is deprecated since Linux 6.1.
> +.RE
> +.TP
> +.I start_ip
> +Instruction pointer address of the first instruction of the sequence of
> +consecutive assembly instructions.
> +.TP
> +.I post_commit_offset
> +Offset (from start_ip address) of the address after the last instruction
> +of the sequence of consecutive assembly instructions.
> +.TP
> +.I abort_ip
> +Instruction pointer address where to move the execution flow in case of
> +abort of the sequence of consecutive assembly instructions.
> +.RE
> +.PP
> +The
> +.I rseq_len
> +argument is the size of the
> +.B struct rseq
> +to register.
> +.PP
> +The
> +.I flags
> +argument is 0 for registration, and
> +.B RSEQ_FLAG_UNREGISTER
> +for unregistration.
> +.PP
> +The
> +.I sig
> +argument is the 32-bit signature to be expected before the abort
> +handler code.
> +.PP
> +A single library per process should keep the
> +.B struct rseq
> +in a per-thread data structure.
> +The
> +.I cpu_id
> +field should be initialized to -1, and the
> +.I cpu_id_start
> +field should be initialized to a possible CPU value (typically 0).
> +.PP
> +Each thread is responsible for registering and unregistering its
> +.BR "struct rseq" .
> +No more than one
> +.B struct rseq
> +address can be registered per thread at a given time.
> +.PP
> +Reclaim of
> +.B struct rseq
> +object's memory must only be done after either an explicit rseq
> +unregistration is performed or after the thread exits.
> +.PP
> +In a typical usage scenario, the thread registering the
> +.B struct rseq
> +will be performing loads and stores from/to that structure.
> +It is however also allowed to read that structure from other threads.
> +The
> +.B struct rseq
> +field updates performed by the kernel provide relaxed atomicity
> +semantics (atomic store, without memory ordering),
> +which guarantee that other threads performing relaxed atomic reads
> +(atomic load, without memory ordering) of the cpu number fields will
> +always observe a consistent value.
> +.SH RETURN VALUE
> +A return value of 0 indicates success.
> +On error, \-1 is returned, and
> +.I errno
> +is set appropriately.

Michael tried to add some consistency to the RETURN VALUE (and EXIT STATUS) sections.
Please have a look at `git show 30b0d8d97e5e^..31e9088393e4`.

> +.SH ERRORS
> +.TP
> +.B EINVAL
> +Either
> +.I flags
> +contains an invalid value, or
> +.I rseq
> +contains an address which is not appropriately aligned, or
> +.I rseq_len
> +contains an incorrect size.
> +.TP
> +.B ENOSYS
> +The
> +.BR rseq ()
> +system call is not implemented by this kernel.
> +.TP
> +.B EFAULT
> +.I rseq
> +is an invalid address.
> +.TP
> +.B EBUSY
> +Restartable sequence is already registered for this thread.
> +.TP
> +.B EPERM
> +The
> +.I sig
> +argument on unregistration does not match the signature received
> +on registration.
> +.SH VERSIONS
> +The
> +.BR rseq ()
> +system call was added in Linux 4.18.
> +.SH STANDARDS
> +.BR rseq ()
> +is Linux-specific.
> +.SH SEE ALSO
> +.BR sched_getcpu (3) ,
> +.BR membarrier (2) ,
> +.BR getauxval (3)

Cheers,

Alex

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