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Subject[PATCH] rcu/tree: Add comment to describe GP done condition in fqs loop
Date
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;
--
2.17.1
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