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

[Frederic Weisbecker]

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?