From mboxrd@z Thu Jan  1 00:00:00 1970
From: Steven Rostedt <rostedt@goodmis.org>
Subject: Re: [RFC PATCH for 4.17 02/21] rseq: Introduce restartable
 sequences system call (v12)
Date: Thu, 29 Mar 2018 14:07:52 -0400
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To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Peter Zijlstra <peterz@infradead.org>, Thomas Gleixner <tglx@linutronix.de>, "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>, 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>, Josh Triplett <josh@joshtriplett.org>, Linus Torvalds <torvalds@linux-foundation.org>, Catalin Marinas <catalin.marinas@arm.com>W
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On Thu, 29 Mar 2018 14:02:33 -0400 (EDT)
Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:

> Currently, anyone using ptrace on a process has pretty much given up all
> hopes of performance. Processes will use rseq to gain performance, not the
> opposite, so this deterioration will be unwelcome.

The ptrace path has nothing to do with ptrace anymore, and probably be
hard to notice the performance hit. You simply set a TIF flag, and on
exit of the syscall it jumps to a path that checks special cases
(tracing system calls being one of them). It's called the ptrace path
because ptrace was the first one to use it (I'm guessing, I haven't
actually looked at the history).

This is used to add any system call checks that are not done during
normal operation. And this certainly falls under that category.

-- Steve