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

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Message-ID: <1523662633.2105.1522248474778.JavaMail.zimbra@efficios.com>
Date:   Wed, 28 Mar 2018 10:47:54 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     "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>,
        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>, 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>,
        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 28, 2018, at 8:50 AM, Peter Zijlstra peterz@...radead.org wrote:

> On Tue, Mar 27, 2018 at 12:05:23PM -0400, Mathieu Desnoyers wrote:
>> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
>> index fb5fc458547f..66b070444a7e 100644
>> --- a/kernel/sched/sched.h
>> +++ b/kernel/sched/sched.h
>> @@ -1249,6 +1249,7 @@ static inline void __set_task_cpu(struct task_struct *p,
>> unsigned int cpu)
>>  #endif
>>  	p->wake_cpu = cpu;
>>  #endif
>> +	rseq_migrate(p);
>>  }
> 
> I think you want that in set_task_cpu(), right next to nr_migrations++.

This would miss the __set_task_cpu() call from sched_fork() and wake_up_new_task().
Those cases are not accounted as explicit "migrations", but it does change the CPU
of the current task. So if for some weird reason userspace wants to fork() while in
a rseq critical section, we want to trigger a rseq restart.

Note that rseq_fork() implies rseq_preempt(), but userspace can request to
track only migrations for a given rseq critical section (by using the
RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT flag), so the rseq_preempt() in rseq_fork()
is not enough to restart if a migration between CPUs is done across a fork.

An alternative to this would be to call rseq_migrate() in rseq_fork().

Thoughts ?

Thanks,

Mathieu


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