Re: RCU: Question rcu_preempt_blocked_readers_cgp in rcu_gp_fqs_loop func

Joel Fernandes
2020-09-14T20:06:45Z

On Wed, Sep 09, 2020 at 04:22:41AM -0700, Paul E. McKenney wrote:
> On Wed, Sep 09, 2020 at 07:03:39AM +0000, Zhang, Qiang wrote:
> >
> > When config preempt RCU, and then there are multiple levels node, the current task is preempted in rcu read critical region.
> > the current task be add to "rnp->blkd_tasks" link list, and the "rnp->gp_tasks" may be assigned a value . these rnp is leaf node in RCU tree.
> >
> > But in "rcu_gp_fqs_loop" func, we check blocked readers in root node.
> >
> > static void rcu_gp_fqs_loop(void)
> > {
> > .....
> > struct rcu_node *rnp = rcu_get_root();
> > .....
> > if (!READ_ONCE(rnp->qsmask) &&
> > !rcu_preempt_blocked_readers_cgp(rnp)) ------> rnp is root node
> > break;
> > ....
> > }
> >
> > the root node's blkd_tasks never add task, the "rnp->gp_tasks" is never be assigned value, this check is invailed.
> > Should we check leaf nodes like this
>
> There are two cases:
>
> 1. There is only a single rcu_node structure, which is both root
> and leaf. In this case, the current check is required: Both
> ->qsmask and the ->blkd_tasks list must be checked. Your
> rcu_preempt_blocked_readers() would work in this case, but
> the current code is a bit faster because it does not need
> to acquire the ->lock nor does it need the loop overhead.
>
> 2. There are multiple levels. In this case, as you say, the root
> rcu_node structure's ->blkd_tasks list will always be empty.
> But also in this case, the root rcu_node structure's ->qsmask
> cannot be zero until all the leaf rcu_node structures' ->qsmask
> fields are zero and their ->blkd_tasks lists no longer have
> tasks blocking the current grace period. This means that your
> rcu_preempt_blocked_readers() function would never return
> true in this case.
>
> So the current code is fine.
>
> Are you seeing failures on mainline kernels? If so, what is the failure
> mode?

Nice question! He had me wondering there too for a bit ;-)

Thanks,

- Joel