linux-kernel - Re: [patch 1/2] nohz: only wakeup a single target cpu when kicking a task

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20201008194819.GA86389@lothringen>
Date:   Thu, 8 Oct 2020 21:48:19 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Peter Xu <peterx@...hat.com>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        linux-kernel@...r.kernel.org,
        Nitesh Narayan Lal <nitesh@...hat.com>
Subject: Re: [patch 1/2] nohz: only wakeup a single target cpu when kicking a
 task

On Thu, Oct 08, 2020 at 05:28:44PM +0200, Peter Zijlstra wrote:
> On Thu, Oct 08, 2020 at 10:59:40AM -0400, Peter Xu wrote:
> > On Wed, Oct 07, 2020 at 03:01:52PM -0300, Marcelo Tosatti wrote:
> > > +static void tick_nohz_kick_task(struct task_struct *tsk)
> > > +{
> > > +	int cpu = task_cpu(tsk);
> > > +
> > > +	/*
> > > +	 * If the task concurrently migrates to another cpu,
> > > +	 * we guarantee it sees the new tick dependency upon
> > > +	 * schedule.
> > > +	 *
> > > +	 *
> > > +	 * set_task_cpu(p, cpu);
> > > +	 *   STORE p->cpu = @cpu
> > > +	 * __schedule() (switch to task 'p')
> > > +	 *   LOCK rq->lock
> > > +	 *   smp_mb__after_spin_lock()          STORE p->tick_dep_mask
> > > +	 *   tick_nohz_task_switch()            smp_mb() (atomic_fetch_or())
> > > +	 *      LOAD p->tick_dep_mask           LOAD p->cpu
> > > +	 */
> > > +
> > > +	preempt_disable();
> > 
> > Pure question: is preempt_disable() required here?  Same question to
> > tick_nohz_full_kick_all().
> 
> I think it serializes against hotplug.

Exactly!

Thanks.