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SubjectRe: [PATCH] rcu/tree: Add comment to describe GP done condition in fqs loop
On Fri, Jun 10, 2022 at 04:37:39PM +0000, Joel Fernandes wrote:
> On Thu, Jun 09, 2022 at 12:43:40PM +0530, Neeraj Upadhyay wrote:
> > Add a comment to explain why !rcu_preempt_blocked_readers_cgp() condition
> > is required on root rnp node, for GP completion check in rcu_gp_fqs_loop().
> >
> > Signed-off-by: Neeraj Upadhyay <quic_neeraju@quicinc.com>
> > ---
> > kernel/rcu/tree.c | 12 +++++++++++-
> > 1 file changed, 11 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index a93c5f4d7e09..9cd1ba512fdc 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -2083,7 +2083,17 @@ static noinline_for_stack void rcu_gp_fqs_loop(void)
> > rcu_gp_torture_wait();
> > WRITE_ONCE(rcu_state.gp_state, RCU_GP_DOING_FQS);
> > /* Locking provides needed memory barriers. */
> > - /* If grace period done, leave loop. */
> > + /*
> > + * If grace period done, leave loop. rcu_preempt_blocked_readers_cgp(rnp)
> > + * check is required for the case where we only have single node in the
> > + * rcu_node tree; so, root rcu node is also the leaf node, where readers
> > + * blocking current gp are queued. For multi-node tree, checking ->qsmask
> > + * on the root node is sufficient, as root rcu node's ->qsmask is only
> > + * cleared, when all leaf rcu nodes have propagated their quiescent
> > + * state to their parent node, which happens only after both ->qsmask
> > + * and rcu_preempt_blocked_readers_cgp(rnp_leaf) are cleared for those
> > + * leaf nodes.
> > + */
> > if (!READ_ONCE(rnp->qsmask) &&
> > !rcu_preempt_blocked_readers_cgp(rnp))
> > break;
>
> Paul's wording changes are OK with me.
>
> For the patch:
> Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>

Applied, thank you!

Thanx, Paul

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