linux-kernel - Re: [RFC PATCH v7 1/7] Restartable sequences system call

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Date:	Wed, 10 Aug 2016 17:40:39 +0000 (UTC)
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Boqun Feng <boqun.feng@...il.com>,
	Peter Zijlstra <peterz@...radead.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>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-api <linux-api@...r.kernel.org>,
	Paul Turner <pjt@...gle.com>, Andrew Hunter <ahh@...gle.com>,
	Andi Kleen <andi@...stfloor.org>,
	Dave Watson <davejwatson@...com>, Chris Lameter <cl@...ux.com>,
	Ben Maurer <bmaurer@...com>, rostedt <rostedt@...dmis.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.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>
Subject: Re: [RFC PATCH v7 1/7] Restartable sequences system call

----- On Aug 10, 2016, at 4:01 AM, Andy Lutomirski luto@...capital.net wrote:

> On Tue, Aug 9, 2016 at 9:13 AM, Boqun Feng <boqun.feng@...il.com> wrote:

<snip>

> 
>> However, I'm thinking maybe we can use some tricks to avoid unnecessary
>> aborts-on-preemption.
>>
>> First of all, I notice we haven't make any constraint on what kind of
>> memory objects could be "protected" by rseq critical sections yet. And I
>> think this is something we should decide before adding this feature into
>> kernel.
>>
>> We can do some optimization if we have some constraints. For example, if
>> the memory objects inside the rseq critical sections could only be
>> modified by userspace programs, we therefore don't need to abort
>> immediately when userspace task -> kernel task context switch.
> 
> True, although trying to do a syscall in an rseq critical section
> seems like a bad idea in general.

The scenario above does not require the rseq critical section to perform
an explicit system call. It can happen from simple timer-driven preemption
of user-space.

<snip>

> 
> But do we need to protect MAP_SHARED objects?  If not, maybe we could
> only track context switches between different tasks sharing the same
> mm.

I have tracing use-cases involving MAP_SHARED objects for rseq: per-cpu
buffers.

Moreover, if you only track context switch between tasks with the same
mm, you run into issues if you have:

Process A
  Thread 1 (rseq)
  Thread 2 (rseq)

Process B
  Thread 1

Scheduling: A.1 -> B.1 -> A.2 -> B.1 -> A.1

There is no scheduling between threads of the same process here, but
the entire chain involves two threads of the same process accessing
the same per-cpu data concurrently.

Thanks,

Mathieu


> 
> --Andy

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