linux-kernel - Re: [RFC PATCH for 4.17 02/21] rseq: Introduce restartable sequences system call (v12)

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Date:   Wed, 28 Mar 2018 11:37:06 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Andy Lutomirski <luto@...capital.net>,
        Dave Watson <davejwatson@...com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-api <linux-api@...r.kernel.org>,
        Paul Turner <pjt@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Russell King <linux@....linux.org.uk>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Andrew Hunter <ahh@...gle.com>,
        Andi Kleen <andi@...stfloor.org>, Chris Lameter <cl@...ux.com>,
        Ben Maurer <bmaurer@...com>, rostedt <rostedt@...dmis.org>,
        Josh Triplett <josh@...htriplett.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [RFC PATCH for 4.17 02/21] rseq: Introduce restartable
 sequences system call (v12)

----- On Mar 28, 2018, at 11:28 AM, Peter Zijlstra peterz@...radead.org wrote:

> On Wed, Mar 28, 2018 at 11:14:05AM -0400, Mathieu Desnoyers wrote:
> 
>> > If at all possible I would make it SIGSEGV when issueing SYSCALL()s from
>> > within an RSEQ.
>> 
>> What's the goal there ? rseq critical sections can technically do system calls
>> if they wish. Why prevent this ?
> 
> This all started as a way to do 'small' _fast_ per-cpu ops, System calls
> do NOT fit in that pattern. If you're willing to do a system calls the
> cost of atomics is not a problem.

I'm not arguing that a typical rseq would do a system call. I'm merely
pointing out that if we start putting arbitrary limitations like "SIGSEGV
when a fork or system call is encountered on top of rseq", this will cause
pain in user-space.

> 
>> How would you handle signal handlers that issue system calls while nested
>> on top of a rseq critical section in the userspace thread ? SIGSEGV on
>> SYSCALLs will break this case.
> 
> Have the rseq thing aborted prior to delivering the signal ?

Not if the RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL flag is set either in the TLS
or in the rseq_cs structure.

How about we simply add a rseq_migrate() within rseq_fork() (when
forking to a new process), which will allow me to move the rseq_migrate
from __set_task_cpu() to set_task_cpu() as you request. Is that solution
acceptable for you ?

Thanks,

Mathieu


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