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

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Nov 20, 2018 at 11:40:22PM +0100, Frederic Weisbecker wrote:
> 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?

Yeah; release_task() is not the normal exit path. Oleg can probably
remember how all that works, because I always get lost there :-/

In any case, have a look at task_rcu_dereference(), but that still does
not explain the rcu_assign_pointer() stuff you use to set
kcpustat->curr.