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Subject[PATCH 3/4] rcu: Use better variable names in funnel locking loop
Date
From: "Joel Fernandes (Google)" <joel@joelfernandes.org>

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 rnp, and rename other variables as
well to be more appropriate.

Original patch: https://patchwork.kernel.org/patch/10396577/

Signed-off-by: Joel Fernandes <joel@joelfernandes.org>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
kernel/rcu/tree.c | 52 +++++++++++++++++++++++------------------------
1 file changed, 26 insertions(+), 26 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 0ad61c97da69..31f4b4b7d824 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1526,7 +1526,7 @@ static void trace_rcu_this_gp(struct rcu_node *rnp, struct rcu_data *rdp,

/*
* rcu_start_this_gp - Request the start of a particular grace period
- * @rnp: The leaf node of the CPU from which to start.
+ * @rnp_start: The leaf node of the CPU from which to start.
* @rdp: The rcu_data corresponding to the CPU from which to start.
* @gp_seq_req: The gp_seq of the grace period to start.
*
@@ -1540,12 +1540,12 @@ static void trace_rcu_this_gp(struct rcu_node *rnp, struct rcu_data *rdp,
*
* Returns true if the GP thread needs to be awakened else false.
*/
-static bool rcu_start_this_gp(struct rcu_node *rnp, struct rcu_data *rdp,
+static bool rcu_start_this_gp(struct rcu_node *rnp_start, struct rcu_data *rdp,
unsigned long gp_seq_req)
{
bool ret = false;
struct rcu_state *rsp = rdp->rsp;
- struct rcu_node *rnp_root;
+ struct rcu_node *rnp;

/*
* Use funnel locking to either acquire the root rcu_node
@@ -1556,58 +1556,58 @@ static bool rcu_start_this_gp(struct rcu_node *rnp, struct rcu_data *rdp,
* scan the leaf rcu_node structures. Note that rnp->lock must
* not be released.
*/
- raw_lockdep_assert_held_rcu_node(rnp);
- trace_rcu_this_gp(rnp, rdp, gp_seq_req, 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_req) ||
- rcu_seq_started(&rnp_root->gp_seq, gp_seq_req) ||
- (rnp != rnp_root &&
- rcu_seq_state(rcu_seq_current(&rnp_root->gp_seq)))) {
- trace_rcu_this_gp(rnp_root, rdp, gp_seq_req,
+ raw_lockdep_assert_held_rcu_node(rnp_start);
+ trace_rcu_this_gp(rnp_start, rdp, gp_seq_req, TPS("Startleaf"));
+ for (rnp = rnp_start; 1; rnp = rnp->parent) {
+ if (rnp != rnp_start)
+ raw_spin_lock_rcu_node(rnp);
+ if (ULONG_CMP_GE(rnp->gp_seq_needed, gp_seq_req) ||
+ rcu_seq_started(&rnp->gp_seq, gp_seq_req) ||
+ (rnp != rnp_start &&
+ rcu_seq_state(rcu_seq_current(&rnp->gp_seq)))) {
+ trace_rcu_this_gp(rnp, rdp, gp_seq_req,
TPS("Prestarted"));
goto unlock_out;
}
- rnp_root->gp_seq_needed = gp_seq_req;
- if (rcu_seq_state(rcu_seq_current(&rnp->gp_seq))) {
+ rnp->gp_seq_needed = gp_seq_req;
+ if (rcu_seq_state(rcu_seq_current(&rnp_start->gp_seq))) {
/*
* We just marked the leaf, and a grace period
* is in progress, which means that rcu_gp_cleanup()
* will see the marking. Bail to reduce contention.
*/
- trace_rcu_this_gp(rnp, rdp, gp_seq_req,
+ trace_rcu_this_gp(rnp_start, rdp, gp_seq_req,
TPS("Startedleaf"));
goto unlock_out;
}
- if (rnp_root != rnp && rnp_root->parent != NULL)
- raw_spin_unlock_rcu_node(rnp_root);
- if (!rnp_root->parent)
+ if (rnp != rnp_start && rnp->parent != NULL)
+ raw_spin_unlock_rcu_node(rnp);
+ if (!rnp->parent)
break; /* At root, and perhaps also leaf. */
}

/* If GP already in progress, just leave, otherwise start one. */
if (rcu_gp_in_progress(rsp)) {
- trace_rcu_this_gp(rnp_root, rdp, gp_seq_req, TPS("Startedleafroot"));
+ trace_rcu_this_gp(rnp, rdp, gp_seq_req, TPS("Startedleafroot"));
goto unlock_out;
}
- trace_rcu_this_gp(rnp_root, rdp, gp_seq_req, TPS("Startedroot"));
+ trace_rcu_this_gp(rnp, rdp, gp_seq_req, TPS("Startedroot"));
WRITE_ONCE(rsp->gp_flags, rsp->gp_flags | RCU_GP_FLAG_INIT);
rsp->gp_req_activity = jiffies;
if (!rsp->gp_kthread) {
- trace_rcu_this_gp(rnp_root, rdp, gp_seq_req, TPS("NoGPkthread"));
+ trace_rcu_this_gp(rnp, rdp, gp_seq_req, TPS("NoGPkthread"));
goto unlock_out;
}
trace_rcu_grace_period(rsp->name, READ_ONCE(rsp->gp_seq), TPS("newreq"));
ret = true; /* Caller must wake GP kthread. */
unlock_out:
/* Push furthest requested GP to leaf node and rcu_data structure. */
- if (ULONG_CMP_GE(rnp_root->gp_seq_needed, gp_seq_req)) {
- rnp->gp_seq_needed = gp_seq_req;
+ if (ULONG_CMP_GE(rnp->gp_seq_needed, gp_seq_req)) {
+ rnp_start->gp_seq_needed = gp_seq_req;
rdp->gp_seq_needed = gp_seq_req;
}
- if (rnp != rnp_root)
- raw_spin_unlock_rcu_node(rnp_root);
+ if (rnp != rnp_start)
+ raw_spin_unlock_rcu_node(rnp);
return ret;
}

--
2.17.0.441.gb46fe60e1d-goog
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