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Subject[PATCH RFC 5/8] rcu: Use rcu_node as temporary variable in funnel locking loop
Date
The funnel locking loop in rcu_start_this_gp uses rcu_root as a
temporary variable while walking the combining tree. This causes a
tiresome exercise of a code reader reminding themselves that rcu_root
may not be root. Lets just call it rcu_node, and then finally when
rcu_node is the rcu_root, lets assign it at that time.

Just a clean up patch, no logical change.

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
kernel/rcu/tree.c | 34 ++++++++++++++++++----------------
1 file changed, 18 insertions(+), 16 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 9f5679ba413b..40670047d22c 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1568,7 +1568,7 @@ static bool rcu_start_this_gp(struct rcu_node *rnp, struct rcu_data *rdp,
{
bool ret = false;
struct rcu_state *rsp = rdp->rsp;
- struct rcu_node *rnp_root;
+ struct rcu_node *rnp_node, *rnp_root = NULL;

/*
* Use funnel locking to either acquire the root rcu_node
@@ -1581,24 +1581,26 @@ static bool rcu_start_this_gp(struct rcu_node *rnp, struct rcu_data *rdp,
*/
raw_lockdep_assert_held_rcu_node(rnp);
trace_rcu_this_gp(rnp, rdp, gp_seq_start, TPS("Startleaf"));
- for (rnp_root = rnp; 1; rnp_root = rnp_root->parent) {
- if (rnp_root != rnp)
- raw_spin_lock_rcu_node(rnp_root);
- if (ULONG_CMP_GE(rnp_root->gp_seq_needed, gp_seq_start) ||
- rcu_seq_done(&rnp_root->gp_seq, gp_seq_start) ||
- (rnp != rnp_root &&
- rcu_seq_state(rcu_seq_current(&rnp_root->gp_seq)))) {
- trace_rcu_this_gp(rnp_root, rdp, gp_seq_start,
+ for (rnp_node = rnp; 1; rnp_node = rnp_node->parent) {
+ if (rnp_node != rnp)
+ raw_spin_lock_rcu_node(rnp_node);
+ if (ULONG_CMP_GE(rnp_node->gp_seq_needed, gp_seq_start) ||
+ rcu_seq_done(&rnp_node->gp_seq, gp_seq_start) ||
+ (rnp != rnp_node &&
+ rcu_seq_state(rcu_seq_current(&rnp_node->gp_seq)))) {
+ trace_rcu_this_gp(rnp_node, rdp, gp_seq_start,
TPS("Prestarted"));
goto unlock_out;
}
- rnp_root->gp_seq_needed = gp_seq_start;
+ rnp_node->gp_seq_needed = gp_seq_start;
if (rcu_seq_state(rcu_seq_current(&rnp->gp_seq)))
goto unlock_out;
- if (rnp_root != rnp && rnp_root->parent != NULL)
- raw_spin_unlock_rcu_node(rnp_root);
- if (!rnp_root->parent)
+ if (rnp_node != rnp && rnp_node->parent != NULL)
+ raw_spin_unlock_rcu_node(rnp_node);
+ if (!rnp_node->parent) {
+ rnp_root = rnp_node;
break; /* At root, and perhaps also leaf. */
+ }
}

/* If GP already in progress, just leave, otherwise start one. */
@@ -1616,10 +1618,10 @@ static bool rcu_start_this_gp(struct rcu_node *rnp, struct rcu_data *rdp,
trace_rcu_grace_period(rsp->name, READ_ONCE(rsp->gp_seq), TPS("newreq"));
ret = true; /* Caller must wake GP kthread. */
unlock_out:
- if (rnp != rnp_root)
- raw_spin_unlock_rcu_node(rnp_root);
+ if (rnp != rnp_node)
+ raw_spin_unlock_rcu_node(rnp_node);
/* Push furthest requested GP to leaf node and rcu_data structure. */
- if (ULONG_CMP_GE(rnp_root->gp_seq_needed, gp_seq_start)) {
+ if (ULONG_CMP_GE(rnp_node->gp_seq_needed, gp_seq_start)) {
rnp->gp_seq_needed = gp_seq_start;
rdp->gp_seq_needed = gp_seq_start;
}
--
2.17.0.441.gb46fe60e1d-goog
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