linux-kernel - Re: [PATCH 20/25] sched/kcpustat: Introduce vtime-aware kcpustat accessor

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Date:   Tue, 20 Nov 2018 23:40:22 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Wanpeng Li <wanpengli@...cent.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Yauheni Kaliuta <yauheni.kaliuta@...hat.com>,
        Ingo Molnar <mingo@...nel.org>, Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH 20/25] sched/kcpustat: Introduce vtime-aware kcpustat
 accessor

On Tue, Nov 20, 2018 at 03:23:06PM +0100, Peter Zijlstra wrote:
> On Wed, Nov 14, 2018 at 03:46:04AM +0100, Frederic Weisbecker wrote:
> 
> > +void kcpustat_cputime(struct kernel_cpustat *kcpustat, int cpu,
> > +		      u64 *user, u64 *nice, u64 *system,
> > +		      u64 *guest, u64 *guest_nice)
> > +{
> > +	struct task_struct *curr;
> > +	struct vtime *vtime;
> > +	int err;
> > +
> > +	if (!vtime_accounting_enabled()) {
> > +		kcpustat_cputime_raw(kcpustat, user, nice,
> > +				     system, guest, guest_nice);
> > +		return;
> > +	}
> > +
> > +	rcu_read_lock();
> > +
> > +	do {
> > +		curr = rcu_dereference(kcpustat->curr);
> 
> Like I explained earlier; I don't think the above is correct.
> task_struct is itself not RCU protected.

But there is at least one put_task_struct() that is enqueued as an RCU callback
on release_task(). That patchset (try to) make sure that kcpustat->curr can't
be assigned beyond that point.

Or did I misunderstand something?