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Subject[PATCH tip/core/rcu 3/3] rcu: cleanups for note_new_gpnum() uses of ->gpnum
Date
From: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Imposes a clear locking design on the note_new_gpnum() function's use
of the ->gpnum counter. This is done by updating rdp->gpnum only from
the corresponding leaf rcu_node structure's rnp->gpnum field, and even
then only under the protection of that same rcu_node structure's ->lock
field. Performance and scalability are maintained using a form of
double-checked locking, and excessive spinning is avoided by use of
the spin_trylock() function. The use of spin_trylock() is safe due to
the fact that CPUs who fail to acquire this lock will try again later.
The hierarchical nature of the rcu_node data structure limits contention
(which could be limited further if need be using the RCU_FANOUT kernel
parameter).

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
kernel/rcutree.c | 32 +++++++++++++++++++++++++++-----
1 files changed, 27 insertions(+), 5 deletions(-)

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 383ff74..650f6e3 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -540,13 +540,33 @@ static void check_cpu_stall(struct rcu_state *rsp, struct rcu_data *rdp)
/*
* Update CPU-local rcu_data state to record the newly noticed grace period.
* This is used both when we started the grace period and when we notice
- * that someone else started the grace period.
+ * that someone else started the grace period. The caller must hold the
+ * ->lock of the leaf rcu_node structure corresponding to the current CPU,
+ * and must have irqs disabled.
*/
+static void __note_new_gpnum(struct rcu_state *rsp, struct rcu_node *rnp, struct rcu_data *rdp)
+{
+ if (rdp->gpnum != rnp->gpnum) {
+ rdp->qs_pending = 1;
+ rdp->passed_quiesc = 0;
+ rdp->gpnum = rnp->gpnum;
+ }
+}
+
static void note_new_gpnum(struct rcu_state *rsp, struct rcu_data *rdp)
{
- rdp->qs_pending = 1;
- rdp->passed_quiesc = 0;
- rdp->gpnum = rsp->gpnum;
+ unsigned long flags;
+ struct rcu_node *rnp;
+
+ local_irq_save(flags);
+ rnp = rdp->mynode;
+ if (rdp->gpnum == ACCESS_ONCE(rnp->gpnum) || /* outside lock. */
+ !spin_trylock_irqsave(&rnp->lock, flags)) { /* retry later. */
+ local_irq_restore(flags);
+ return;
+ }
+ __note_new_gpnum(rsp, rnp, rdp);
+ spin_unlock_irqrestore(&rnp->lock, flags);
}

/*
@@ -637,6 +657,9 @@ rcu_start_gp_per_cpu(struct rcu_state *rsp, struct rcu_node *rnp, struct rcu_dat
*/
rdp->nxttail[RCU_NEXT_READY_TAIL] = rdp->nxttail[RCU_NEXT_TAIL];
rdp->nxttail[RCU_WAIT_TAIL] = rdp->nxttail[RCU_NEXT_TAIL];
+
+ /* Set state so that this CPU will detect the next quiescent state. */
+ __note_new_gpnum(rsp, rnp, rdp);
}

/*
@@ -664,7 +687,6 @@ rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
rsp->jiffies_force_qs = jiffies + RCU_JIFFIES_TILL_FORCE_QS;
record_gp_stall_check_time(rsp);
dyntick_record_completed(rsp, rsp->completed - 1);
- note_new_gpnum(rsp, rdp);

/* Special-case the common single-level case. */
if (NUM_RCU_NODES == 1) {
--
1.5.2.5


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