Date: Mon, 15 Apr 2013 18:09:08 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Robin Holt <holt@sgi.com>
Cc: "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>,
        Ingo Molnar <mingo@kernel.org>, Paul Mackerras <paulus@samba.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Russ Anderson <rja@sgi.com>, Shawn Guo <shawn.guo@linaro.org>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        the arch/x86 maintainers <x86@kernel.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Tejun Heo <tj@kernel.org>, Lai Jiangshan <laijs@cn.fujitsu.com>,
        Michel Lespinasse <walken@google.com>,
        "rusty@rustcorp.com.au" <rusty@rustcorp.com.au>,
        Peter Zijlstra <peterz@infradead.org>
Subject: Re: Bulk CPU Hotplug (Was Re: [PATCH] Do not force shutdown/reboot
	to boot cpu.)
Message-ID: <20130415160908.GA17950@redhat.com>
References: <20130410111620.GB29752@gmail.com> <CA+55aFw8bRwMRm8cWtTGRvd1AEP-LR7pYL-pEoBkHqJUuJrjSg@mail.gmail.com> <20130411053106.GA9042@drongo> <5166B05E.8010904@linux.vnet.ibm.com> <20130411134837.GE3672@sgi.com> <20130412053718.GC3887@gmail.com> <5167A52F.6020503@linux.vnet.ibm.com> <20130412093149.GT3658@sgi.com> <20130413163022.GA7500@redhat.com> <20130415160408.GA3658@sgi.com>
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On 04/15, Robin Holt wrote:
>
> On Sat, Apr 13, 2013 at 06:30:22PM +0200, Oleg Nesterov wrote:
> > On 04/12, Robin Holt wrote:
> > >
> > > +void migrate_to_boot_cpu(void)
> > > +{
> > > +	/* The boot cpu is always logical cpu 0 */
> > > +	int reboot_cpu_id = 0;
> > > +
> > > +	/* Make certain the cpu I'm about to reboot on is online */
> > > +	if (!cpu_online(reboot_cpu_id))
> > > +		reboot_cpu_id = smp_processor_id();
> > > +
> > > +	/* Make certain I only run on the appropriate processor */
> > > +	set_cpus_allowed_ptr(current, cpumask_of(reboot_cpu_id));
> >
> > This is only theoretical, but perhaps it makes sense to set
> > PF_THREAD_BOUND before set_cpus_allowed_ptr() ? To prevent the
> > race with another thread doing sched_setaffinity().
>
> I don't quite understand this comment.  We are migrating our own thread.
> How does setting PF_THREAD_BOUND have any effect?

Suppose that another thread does, say, sys_sched_setaffinity(our_pid)
right after set_cpus_allowed_ptr(). If we set PF_THREAD_BOUND the
next set_cpus_allowed() will fail.

Oleg.

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