srcu: Implement more-efficient reader counts

SRCU uses two per-cpu counters: a nesting counter to count the number of
active critical sections, and a sequence counter to ensure that the nesting
counters don't change while they are being added together in
srcu_readers_active_idx_check().

This patch instead uses per-cpu lock and unlock counters. Because both
counters only increase and srcu_readers_active_idx_check() reads the unlock
counter before the lock counter, this achieves the same end without having
to increment two different counters in srcu_read_lock(). This also saves a
smp_mb() in srcu_readers_active_idx_check().

Possible bug: There is no guarantee that the lock counter won't overflow
during srcu_readers_active_idx_check(), as there are no memory barriers
around srcu_flip() (see comment in srcu_readers_active_idx_check() for
details). However, this problem was already present before this patch.

Suggested-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Lance Roy <ldr709@gmail.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 17 insertions, 2 deletions

diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
index 87c5122..d81345b 100644
--- a/kernel/rcu/rcutorture.c
+++ b/kernel/rcu/rcutorture.c
@@ -564,10 +564,25 @@ static void srcu_torture_stats(void)
 	pr_alert("%s%s per-CPU(idx=%d):",
 		 torture_type, TORTURE_FLAG, idx);
 	for_each_possible_cpu(cpu) {
+		unsigned long l0, l1;
+		unsigned long u0, u1;
 		long c0, c1;
+		struct srcu_array *counts = per_cpu_ptr(srcu_ctlp->per_cpu_ref, cpu);
 
-		c0 = (long)per_cpu_ptr(srcu_ctlp->per_cpu_ref, cpu)->c[!idx];
-		c1 = (long)per_cpu_ptr(srcu_ctlp->per_cpu_ref, cpu)->c[idx];
+		u0 = counts->unlock_count[!idx];
+		u1 = counts->unlock_count[idx];
+
+		/*
+		 * Make sure that a lock is always counted if the corresponding
+		 * unlock is counted.
+		 */
+		smp_rmb();
+
+		l0 = counts->lock_count[!idx];
+		l1 = counts->lock_count[idx];
+
+		c0 = l0 - u0;
+		c1 = l1 - u1;
 		pr_cont(" %d(%ld,%ld)", cpu, c0, c1);
 	}
 	pr_cont("\n");