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

[Peter Zijlstra]

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.

> +		if (!curr) {
> +			kcpustat_cputime_raw(kcpustat, user, nice,
> +					     system, guest, guest_nice);
> +			break;
> +		}
> +
> +		vtime = &curr->vtime;
> +		err = kcpustat_vtime(kcpustat, vtime, cpu, user,
> +				     nice, system, guest, guest_nice);
> +	} while (err == -EAGAIN);
> +
> +	rcu_read_unlock();
> +}
> +
>  #endif /* CONFIG_VIRT_CPU_ACCOUNTING_GEN */
> -- 
> 2.7.4
>