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Date:   Wed, 28 Mar 2018 19:49:35 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc:     "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Andy Lutomirski <luto@amacapital.net>,
        Dave Watson <davejwatson@fb.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        linux-api <linux-api@vger.kernel.org>,
        Paul Turner <pjt@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Russell King <linux@arm.linux.org.uk>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Andrew Hunter <ahh@google.com>,
        Andi Kleen <andi@firstfloor.org>, Chris Lameter <cl@linux.com>,
        Ben Maurer <bmaurer@fb.com>, rostedt <rostedt@goodmis.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Michael Kerrisk <mtk.manpages@gmail.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>
Subject: Re: [RFC PATCH for 4.17 02/21] rseq: Introduce restartable sequences
 system call (v12)
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On Wed, Mar 28, 2018 at 11:37:06AM -0400, Mathieu Desnoyers wrote:
> ----- On Mar 28, 2018, at 11:28 AM, Peter Zijlstra peterz@infradead.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.

I don't think disallowing system calls is arbitrary. And I think that is
something we really want to enforce, because it's batshit insane to
allow.

And if we allow now, people _will_ use it and we can't ever take it
away again.