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

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Message-ID: <21903915.856.1522346553810.JavaMail.zimbra@efficios.com>
Date:   Thu, 29 Mar 2018 14:02:33 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     rostedt <rostedt@...dmis.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "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>,
        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>,
        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 29, 2018, at 12:24 PM, rostedt rostedt@...dmis.org wrote:

> On Thu, 29 Mar 2018 11:39:00 -0400 (EDT)
> Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:
> 
>> Enforcing SIGSEGV on syscall entry when nested in a rseq critical section
>> will not be free both in terms of syscall overhead, and in terms of code
>> maintenance: we'd need to add those checks into entry.S for each architecture
>> supported, which pretty much doubles the amount of architecture-specific
>> code we need to implement for rseq. Currently, all we need is to hook in
>> signal delivery and wire up the system call numbers.
> 
> Why not have the check on syscall exit? Then we could use the ptrace
> type mechanism to only go that path when a rseq exists for the program.

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.

One thing I would find more acceptable is to only compile in this check under
a CONFIG_DEBUG_RSEQ option or something similar. This means we can then put
the check at the most convenient location without caring too much about its
performance impact.

Thoughts ?

Thanks,

Mathieu

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