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

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

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

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Re: [RFC PATCH v7 1/7] Restartable sequences system call Andy Lutomirski <luto@amacapital.net> - 2016-07-26 01:10 +0200
  Re: [RFC PATCH v7 1/7] Restartable sequences system call Mathieu Desnoyers <mathieu.desnoyers@efficios.com> - 2016-07-26 05:10 +0200
    Re: [RFC PATCH v7 1/7] Restartable sequences system call Peter Zijlstra <peterz@infradead.org> - 2016-08-03 14:30 +0200
      Re: [RFC PATCH v7 1/7] Restartable sequences system call Andy Lutomirski <luto@amacapital.net> - 2016-08-03 18:50 +0200
        Re: [RFC PATCH v7 1/7] Restartable sequences system call Christoph Lameter <cl@linux.com> - 2016-08-03 20:40 +0200
          Re: [RFC PATCH v7 1/7] Restartable sequences system call Andy Lutomirski <luto@amacapital.net> - 2016-08-04 07:10 +0200
        Re: [RFC PATCH v7 1/7] Restartable sequences system call Boqun Feng <boqun.feng@gmail.com> - 2016-08-04 06:30 +0200
          Re: [RFC PATCH v7 1/7] Restartable sequences system call Andy Lutomirski <luto@amacapital.net> - 2016-08-04 07:20 +0200
            Re: [RFC PATCH v7 1/7] Restartable sequences system call Boqun Feng <boqun.feng@gmail.com> - 2016-08-09 18:20 +0200
              Re: [RFC PATCH v7 1/7] Restartable sequences system call Mathieu Desnoyers <mathieu.desnoyers@efficios.com> - 2016-08-10 20:20 +0200
              Re: [RFC PATCH v7 1/7] Restartable sequences system call Andy Lutomirski <luto@amacapital.net> - 2016-08-10 21:10 +0200
              Re: [RFC PATCH v7 1/7] Restartable sequences system call Mathieu Desnoyers <mathieu.desnoyers@efficios.com> - 2016-08-10 22:50 +0200
            Re: [RFC PATCH v7 1/7] Restartable sequences system call Andy Lutomirski <luto@amacapital.net> - 2016-08-10 21:40 +0200
    Re: [RFC PATCH v7 1/7] Restartable sequences system call Christoph Lameter <cl@linux.com> - 2016-08-03 20:40 +0200
      Re: [RFC PATCH v7 1/7] Restartable sequences system call Mathieu Desnoyers <mathieu.desnoyers@efficios.com> - 2016-08-10 22:10 +0200
        Re: [RFC PATCH v7 1/7] Restartable sequences system call Christoph Lameter <cl@linux.com> - 2016-08-10 22:40 +0200

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