lkml.org 
[lkml]   [2016]   [Aug]   [10]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [RFC PATCH v7 1/7] Restartable sequences system call
----- 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

\
 
 \ /
  Last update: 2016-08-10 23:01    [W:0.138 / U:0.080 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site