Re: [PATCH man-pages] Add rseq manpage

Michael Kerrisk (man-pages)
2019-02-28T08:43:06Z

On 12/6/18 3:42 PM, Mathieu Desnoyers wrote:
> [ Michael, rseq(2) was merged into 4.18. Can you have a look at this
> patch which adds rseq documentation to the man-pages project ? ]
Hi Matthieu

Sorry for the long delay. I've merged this page into a private
branch and have done quite a lot of editing. I have many
questions :-).

In the first instance, I think it is probably best to have
a free-form text discussion rather than firing patches
back and forward. Could you take a look at the questions below
and respond?

Thanks,

Michael

RSEQ(2) Linux Programmer's Manual RSEQ(2)

NAME
rseq - Restartable sequences and CPU number cache

SYNOPSIS
#include <linux/rseq.h>

int rseq(struct rseq *rseq, uint32_t rseq_len, int flags, uint32_t sig);

DESCRIPTION
┌─────────────────────────────────────────────────────┐
│FIXME │
├─────────────────────────────────────────────────────┤
│Imagine you are someone who is pretty new to this │
│idea... What is notably lacking from this page is │
│an overview explaining: │
│ │
│ * What a restartable sequence actually is. │
│ │
│ * An outline of the steps to perform when using │
│ restartable sequences / rseq(2). │
│ │
│I.e., something along the lines of Jon Corbet's │
│https://lwn.net/Articles/697979/. Can you come up │
│with something? (Part of it might be at the start of │
│this page, and the rest in NOTES; it need not be all │
│in one place.) │
└─────────────────────────────────────────────────────┘
The 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.

It allows user-space to perform update operations on per-CPU data with‐
out requiring heavy-weight atomic operations.

┌─────────────────────────────────────────────────────┐
│FIXME │
├─────────────────────────────────────────────────────┤
│In the following para: "a hardware execution con‐ │
│text"? What is the contrast being drawn here? It │
│would be good to state it more explicitly. │
└─────────────────────────────────────────────────────┘
The term CPU used in this documentation refers to a hardware execution
context.

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.

┌─────────────────────────────────────────────────────┐
│FIXME │
├─────────────────────────────────────────────────────┤
│In the preceding sentence, we need a definition of │
│"current CPU". │
└─────────────────────────────────────────────────────┘

┌─────────────────────────────────────────────────────┐
│FIXME │
├─────────────────────────────────────────────────────┤
│In the following, does "It is" means "Restartable │
│sequences are"? │
└─────────────────────────────────────────────────────┘
It is suited for update operations on per-CPU data.

┌─────────────────────────────────────────────────────┐
│FIXME │
├─────────────────────────────────────────────────────┤
│In the following, does "It is" means "Restartable │
│sequences are"? │
└─────────────────────────────────────────────────────┘
It can be used on data structures shared between threads within a
process, and on data structures shared between threads across different
processes.

Some examples of operations that can be accelerated or improved by this
ABI:

· Memory allocator per-CPU free-lists

· Querying the current CPU number

· Incrementing per-CPU counters

· Modifying data protected by per-CPU spinlocks

· Inserting/removing elements in per-CPU linked-lists

· Writing/reading per-CPU ring buffers content

· Accurately reading performance monitoring unit counters with respect
to thread migration

Restartable sequences must not perform system calls. Doing so may
result in termination of the process by a segmentation fault.

The rseq argument is a pointer to the thread-local rseq structure to be
shared between kernel and user-space. The layout of this structure is
shown below.

The rseq_len argument is the size of the struct rseq to register.

The flags argument is 0 for registration, or RSEQ_FLAG_UNREGISTER for
unregistration.

The sig argument is the 32-bit signature to be expected before the
abort handler code.

The rseq structure
The struct rseq is aligned on a 32-byte boundary. This structure is
extensible. Its size is passed as parameter to the rseq() system call.

┌─────────────────────────────────────────────────────┐
│FIXME │
├─────────────────────────────────────────────────────┤
│Below, I added the structure definition (in abbrevi‐ │
│ated form). Is there any reason not to do this? │
└─────────────────────────────────────────────────────┘

struct rseq {
__u32 cpu_id_start;
__u32 cpu_id;
union {
__u64 ptr64;
#ifdef __LP64__
__u64 ptr;
#else
....
#endif
} rseq_cs;
__u32 flags;
} __attribute__((aligned(4 * sizeof(__u64))));

┌─────────────────────────────────────────────────────┐
│FIXME │
├─────────────────────────────────────────────────────┤
│In the text below, I think it would be helpful to │
│explicitly note which of these fields are set by the │
│kernel (on return from the reseq() call) and which │
│are set by the caller (before calling rseq()). Is │
│the following correct: │
│ │
│ cpu_id_start - initialized by caller to possible │
│ CPU number (e.g., 0), updated by kernel │
│ on return │
│ │
│ cpu_id - initialized to -1 by caller, │
│ updated by kernel on return │
│ │
│ rseq_cs - initialized by caller, either to NULL │
│ or a pointer to an 'rseq_cs' structure │
│ that is initialized by the caller │
│ │
│ flags - initialized by caller, used by kernel │
└─────────────────────────────────────────────────────┘

The structure fields are as follows:

┌─────────────────────────────────────────────────────┐
│FIXME │
├─────────────────────────────────────────────────────┤
│In the following paragraph, and in later places, I │
│changed "current thread" to "calling thread". Okay? │
└─────────────────────────────────────────────────────┘

cpu_id_start
Optimistic cache of the CPU number on which the calling thread
is running. The value in this field is guaranteed to always be
a possible CPU number, even when rseq is not initialized. The
value it contains should always be confirmed by reading the
cpu_id field.

┌─────────────────────────────────────────────────────┐
│FIXME │
├─────────────────────────────────────────────────────┤
│What does the last sentence mean? │
└─────────────────────────────────────────────────────┘

This field is an optimistic cache in the sense that it is always
guaranteed to hold a valid CPU number in the range [0..(nr_pos‐
sible_cpus - 1)]. 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.

For user-space applications executed on a kernel without 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 rseq support, that cpu_id field
stays initialized at -1, so the comparison always fails, as
intended. It is then up to user-space to use a fall-back mecha‐
nism, considering that rseq is not available.

┌─────────────────────────────────────────────────────┐
│FIXME │
├─────────────────────────────────────────────────────┤
│The last sentence is rather difficult to grok. Can │
│we say some more here? │
└─────────────────────────────────────────────────────┘

cpu_id Cache of the CPU number on which the calling thread is running.
-1 if uninitialized.

rseq_cs
The rseq_cs field is a pointer to a struct rseq_cs (described
below). It is NULL when no rseq assembly block critical section
is active for the calling thread. Setting it to point to a
critical section descriptor (struct rseq_cs) marks the beginning
of the critical section.

flags Flags indicating the restart behavior for the calling thread.
This is mainly used for debugging purposes. Can be either:

RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT

RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL

RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE

┌─────────────────────────────────────────────────────┐
│FIXME │
├─────────────────────────────────────────────────────┤
│Each of the above values needs an explanation. │
│ │
│Is it correct that only one of the values may be │
│specified in 'flags'? I ask because in the 'rseq_cs' │
│structure below, the 'flags' field is a bit mask │
│where any combination of these flags may be ORed │
│together. │
│ │
└─────────────────────────────────────────────────────┘

The rseq_cs structure
The struct rseq_cs is aligned on a 32-byte boundary and has a fixed
size of 32 bytes.

┌─────────────────────────────────────────────────────┐
│FIXME │
├─────────────────────────────────────────────────────┤
│Below, I added the structure definition (in abbrevi‐ │
│ated form). Is there any reason not to do this? │
└─────────────────────────────────────────────────────┘

struct rseq_cs {
__u32 version;
__u32 flags;
__u64 start_ip;
__u64 post_commit_offset;
__u64 abort_ip;
} __attribute__((aligned(4 * sizeof(__u64))));

The structure fields are as follows:

version
Version of this structure.

┌─────────────────────────────────────────────────────┐
│FIXME │
├─────────────────────────────────────────────────────┤
│What does 'version' need to be initialized to? │
└─────────────────────────────────────────────────────┘

flags Flags indicating the restart behavior of this structure. Can be
a combination of:

RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT

RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL

RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE

┌─────────────────────────────────────────────────────┐
│FIXME │
├─────────────────────────────────────────────────────┤
│Each of the above values needs an explanation. │
└─────────────────────────────────────────────────────┘

start_ip
Instruction pointer address of the first instruction of the
sequence of consecutive assembly instructions.

post_commit_offset
Offset (from start_ip address) of the address after the last
instruction of the sequence of consecutive assembly instruc‐
tions.

abort_ip
Instruction pointer address where to move the execution flow in
case of abort of the sequence of consecutive assembly instruc‐
tions.

NOTES
A single library per process should keep the rseq structure in a
thread-local storage variable. The cpu_id field should be initialized
to -1, and the cpu_id_start field should be initialized to a possible
CPU value (typically 0).

Each thread is responsible for registering and unregistering its rseq
structure. No more than one rseq structure address can be registered
per thread at a given time.

┌─────────────────────────────────────────────────────┐
│FIXME │
├─────────────────────────────────────────────────────┤
│In the following paragraph, what is the difference │
│between "freed" and "reclaim"? I'm supposing they │
│mean the same thing, but it's not clear. And if they │
│do mean the same thing, then the first two sentences │
│appear to contain contradictory information. │
└─────────────────────────────────────────────────────┘

Memory of a registered rseq object must not be freed before the thread
exits. Reclaim of rseq object's memory must only be done after either
an explicit rseq unregistration is performed or after the thread exits.
Keep in mind that the implementation of the Thread-Local Storage (C
language __thread) lifetime does not guarantee existence of the TLS
area up until the thread exits.

In a typical usage scenario, the thread registering the rseq structure
will be performing loads and stores from/to that structure. It is how‐
ever also allowed to read that structure from other threads. The rseq
field updates performed by the kernel provide relaxed atomicity seman‐
tics, which guarantee that other threads performing relaxed atomic
reads of the CPU number cache will always observe a consistent value.

┌─────────────────────────────────────────────────────┐
│FIXME │
├─────────────────────────────────────────────────────┤
│In the preceding paragraph, can we reasonably add │
│some words to explain "relaxed atomicity semantics" │
│and "relaxed atomic reads"? │
└─────────────────────────────────────────────────────┘

RETURN VALUE
A return value of 0 indicates success. On error, -1 is returned, and
errno is set appropriately.

ERRORS
EBUSY Restartable sequence is already registered for this thread.

EFAULT rseq is an invalid address.

EINVAL Either flags contains an invalid value, or rseq contains an
address which is not appropriately aligned, or rseq_len contains
a size that does not match the size received on registration.

┌─────────────────────────────────────────────────────┐
│FIXME │
├─────────────────────────────────────────────────────┤
│The last case "rseq_len contains a size that does │
│not match the size received on registration" can │
│occur only on RSEQ_FLAG_UNREGISTER, tight? │
└─────────────────────────────────────────────────────┘

ENOSYS The rseq() system call is not implemented by this kernel.

EPERM The sig argument on unregistration does not match the signature
received on registration.

VERSIONS
The rseq() system call was added in Linux 4.18.

┌─────────────────────────────────────────────────────┐
│FIXME │
├─────────────────────────────────────────────────────┤
│What is the current state of library support? │
└─────────────────────────────────────────────────────┘

CONFORMING TO
rseq() is Linux-specific.

┌─────────────────────────────────────────────────────┐
│FIXME │
├─────────────────────────────────────────────────────┤
│Is there any example code that can reasonably be │
│included in this manual page? Or some example code │
│that can be referred to? │
└─────────────────────────────────────────────────────┘

SEE ALSO
sched_getcpu(3), membarrier(2)

--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/