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Date:   Wed, 28 Mar 2018 16:59:46 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc:     "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Andy Lutomirski <luto@amacapital.net>,
        Dave Watson <davejwatson@fb.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        linux-api <linux-api@vger.kernel.org>,
        Paul Turner <pjt@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Russell King <linux@arm.linux.org.uk>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Andrew Hunter <ahh@google.com>,
        Andi Kleen <andi@firstfloor.org>, Chris Lameter <cl@linux.com>,
        Ben Maurer <bmaurer@fb.com>, rostedt <rostedt@goodmis.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Michael Kerrisk <mtk.manpages@gmail.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>
Subject: Re: [RFC PATCH for 4.17 02/21] rseq: Introduce restartable sequences
 system call (v12)
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On Wed, Mar 28, 2018 at 10:47:54AM -0400, Mathieu Desnoyers wrote:
> ----- On Mar 28, 2018, at 8:50 AM, Peter Zijlstra peterz@infradead.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().

Correct; but since those are _new_ tasks they _SHOULD_ not have an
active RSEQ to begin with.

> 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.

If at all possible I would make it SIGSEGV when issueing SYSCALL()s from
within an RSEQ.

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

Yes, don't try and support that at all. It's _insane_.