linux-kernel - Re: [PATCH v2 1/8] RISC-V: Do not wait indefinitely in __cpu

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Message-ID: <20190118072038.GA5171@infradead.org>
Date:   Thu, 17 Jan 2019 23:20:38 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Atish Patra <atish.patra@....com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Patrick Stählin <me@...ki.ch>,
        Albert Ou <aou@...s.berkeley.edu>,
        Jason Cooper <jason@...edaemon.net>,
        Alan Kao <alankao@...estech.com>,
        Dmitriy Cherkasov <dmitriy@...-tech.org>,
        Anup Patel <anup@...infault.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Michael Clark <michaeljclark@....com>,
        Marc Zyngier <marc.zyngier@....com>,
        Palmer Dabbelt <palmer@...ive.com>,
        Andreas Schwab <schwab@...e.de>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Zong Li <zongbox@...il.com>
Subject: Re: [PATCH v2 1/8] RISC-V: Do not wait indefinitely in __cpu_up

On Thu, Jan 17, 2019 at 06:35:39PM -0800, Atish Patra wrote:
> On 1/15/19 5:51 AM, Christoph Hellwig wrote:
> > >   void *__cpu_up_stack_pointer[NR_CPUS];
> > >   void *__cpu_up_task_pointer[NR_CPUS];
> > > +static DECLARE_COMPLETION(cpu_running);
> > >   void __init smp_prepare_boot_cpu(void)
> > >   {
> > > @@ -81,6 +82,7 @@ void __init setup_smp(void)
> > >   int __cpu_up(unsigned int cpu, struct task_struct *tidle)
> > >   {
> > > +	int ret = 0;
> > >   	int hartid = cpuid_to_hartid_map(cpu);
> > >   	tidle->thread_info.cpu = cpu;
> > > @@ -96,10 +98,15 @@ int __cpu_up(unsigned int cpu, struct task_struct *tidle)
> > >   		  task_stack_page(tidle) + THREAD_SIZE);
> > >   	WRITE_ONCE(__cpu_up_task_pointer[hartid], tidle);
> > > -	while (!cpu_online(cpu))
> > > -		cpu_relax();
> > > +	wait_for_completion_timeout(&cpu_running,
> > > +					    msecs_to_jiffies(1000));
> > 
> > Having a global completion here worries me.  I bet we have some higher
> > level serialization, but can we comment or even better lockdep assert on
> > that?
> > 
> 
> Yes. It is serialized from smp.c in smp_init(). It brings one cpu online
> at a time for preset_cpu mask.
> 
> Do we still need a lockdep assert ?

I guess the real lock is through cpu_hotplug_lock.  And yes, a comment
or even better lockdep assert would be good.