lkml.org 
[lkml]   [2018]   [Jun]   [13]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: Restartable Sequences system call merged into Linux
----- On Jun 13, 2018, at 7:48 AM, heiko carstens heiko.carstens@de.ibm.com wrote:

> On Mon, Jun 11, 2018 at 03:49:18PM -0400, Mathieu Desnoyers wrote:
>> Hi!
>>
>> Good news! The restartable sequences (rseq) system call is now merged into the
>> master
>> branch of the Linux kernel within the 4.18 merge window:
>>
>> https://github.com/torvalds/linux/commit/d82991a8688ad128b46db1b42d5d84396487a508
>>
>> It would be important to discuss how we should proceed to integrate the library
>> part
>> of rseq (see tools/testing/selftests/rseq/rseq*.{ch}) into glibc, or if it
>> should
>> live in a standalone project.
>
> Is there any documentation available of what is the exact semantics of the
> functions that have to be implemented for additional architectures?

It's documented on top of kernel/rseq.c:

/*
*
* Restartable sequences are a lightweight interface that allows
* user-level code to be executed atomically relative to scheduler
* preemption and signal delivery. Typically used for implementing
* per-cpu operations.
*
* It allows user-space to perform update operations on per-cpu data
* without requiring heavy-weight atomic operations.
*
* Detailed algorithm of rseq user-space assembly sequences:
*
* init(rseq_cs)
* cpu = TLS->rseq::cpu_id_start
* [1] TLS->rseq::rseq_cs = rseq_cs
* [start_ip] ----------------------------
* [2] if (cpu != TLS->rseq::cpu_id)
* goto abort_ip;
* [3] <last_instruction_in_cs>
* [post_commit_ip] ----------------------------
*
* The address of jump target abort_ip must be outside the critical
* region, i.e.:
*
* [abort_ip] < [start_ip] || [abort_ip] >= [post_commit_ip]
*
* Steps [2]-[3] (inclusive) need to be a sequence of instructions in
* userspace that can handle being interrupted between any of those
* instructions, and then resumed to the abort_ip.
*
* 1. Userspace stores the address of the struct rseq_cs assembly
* block descriptor into the rseq_cs field of the registered
* struct rseq TLS area. This update is performed through a single
* store within the inline assembly instruction sequence.
* [start_ip]
*
* 2. Userspace tests to check whether the current cpu_id field match
* the cpu number loaded before start_ip, branching to abort_ip
* in case of a mismatch.
*
* If the sequence is preempted or interrupted by a signal
* at or after start_ip and before post_commit_ip, then the kernel
* clears TLS->__rseq_abi::rseq_cs, and sets the user-space return
* ip to abort_ip before returning to user-space, so the preempted
* execution resumes at abort_ip.
*
* 3. Userspace critical section final instruction before
* post_commit_ip is the commit. The critical section is
* self-terminating.
* [post_commit_ip]
*
* 4. <success>
*
* On failure at [2], or if interrupted by preempt or signal delivery
* between [1] and [3]:
*
* [abort_ip]
* F1. <failure>
*/

> I could look at rseq-skip.h and e.g. rseq-x86.h and try to figure out what
> would be the correct implementation for s390. But having that somewhere
> written down, e.g. as comments in one of the implementations, would be very
> helpful.

The first architecture implemented was rseq-x86.h. Boqun derived rseq-ppc.h
from it, and I derived rseq-arm.h from it. Feel free to start from whichever
architecture has the instruction set which is most similar to yours.

Thanks!

Mathieu

--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com

\
 
 \ /
  Last update: 2018-06-13 18:14    [W:0.055 / U:1.092 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site