linux-kernel - Re: [RFC PATCH v8 1/9] Restartable sequences system call

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Message-ID: <CAPM31RJENYEjctuQ89u8Yv2vvW-3e2CccYk-NAqkdq85smHxUA@mail.gmail.com>
Date:   Sat, 26 Nov 2016 15:43:06 -0800
From:   Paul Turner <pjt@...nel.org>
To:     Pavel Machek <pavel@....cz>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Andy Lutomirski <luto@...capital.net>,
        Dave Watson <davejwatson@...com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        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>,
        Steven 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>
Subject: Re: [RFC PATCH v8 1/9] Restartable sequences system call

On Sat, Aug 27, 2016 at 5:21 AM, Pavel Machek <pavel@....cz> wrote:
>
> Hi!
>
>> Expose a new system call allowing each thread to register one userspace
>> memory area to be used as an ABI between kernel and user-space for two
>> purposes: user-space restartable sequences and quick access to read the
>> current CPU number value from user-space.
>>
>> * Restartable sequences (per-cpu atomics)
>>
>> Restartables sequences allow user-space to perform update operations on
>> per-cpu data without requiring heavy-weight atomic operations.
>>
>> The restartable critical sections (percpu atomics) work has been started
>> by Paul Turner and Andrew Hunter. It lets the kernel handle restart of
>> critical sections. [1] [2] The re-implementation proposed here brings a
>> few simplifications to the ABI which facilitates porting to other
>> architectures and speeds up the user-space fast path. A locking-based
>> fall-back, purely implemented in user-space, is proposed here to deal
>> with debugger single-stepping. This fallback interacts with rseq_start()
>> and rseq_finish(), which force retries in response to concurrent
>> lock-based activity.
>
> Hmm. Purely software fallback needed for singlestepping... Looks like this is malware
> writer's dream come true...
>
> Also if you ever get bug in the restartable code, debugger will be useless to debug it...
> unless new abilities are added to debuggers to manually schedule threads on CPUs.
>
> Is this good idea?

We've had some off-list discussion.

I have a revised version which incoprorates some of Mattheiu's
improvements, but avoids this requirement nearly ready for posting.

- Paul