[PATCH 2/3 RFC paul/rcu/srcu] srcu: only increase the upper bit for srcu_read_lock()

[Lai Jiangshan]

The algorithm/smp_mb()s ensure 'there is only one srcu_read_lock()
between flip and recheck for a cpu'.
Increment of the upper bit for srcu_read_lock() only can
ensure a single pair of lock/unlock change the cpu counter.

Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxx>
---
 include/linux/srcu.h |    2 +-
 kernel/srcu.c        |   11 +++++------
 2 files changed, 6 insertions(+), 7 deletions(-)

diff --git a/include/linux/srcu.h b/include/linux/srcu.h
index a478c8e..5b49d41 100644
--- a/include/linux/srcu.h
+++ b/include/linux/srcu.h
@@ -35,7 +35,7 @@ struct srcu_struct_array {
 };
 
 /* Bit definitions for field ->c above and ->snap below. */
-#define SRCU_USAGE_BITS		2
+#define SRCU_USAGE_BITS		1
 #define SRCU_REF_MASK		(ULONG_MAX >> SRCU_USAGE_BITS)
 #define SRCU_USAGE_COUNT	(SRCU_REF_MASK + 1)
 
diff --git a/kernel/srcu.c b/kernel/srcu.c
index 17e95bc..a51ac48 100644
--- a/kernel/srcu.c
+++ b/kernel/srcu.c
@@ -138,10 +138,10 @@ static bool srcu_readers_active_idx_check(struct srcu_struct *sp, int idx)
 
 	/*
 	 * Now, we check the ->snap array that srcu_readers_active_idx()
-	 * filled in from the per-CPU counter values.  Since both
-	 * __srcu_read_lock() and __srcu_read_unlock() increment the
-	 * upper bits of the per-CPU counter, an increment/decrement
-	 * pair will change the value of the counter.  Since there is
+	 * filled in from the per-CPU counter values. Since
+	 * __srcu_read_lock() increments the upper bits of
+	 * the per-CPU counter, an increment/decrement pair will
+	 * change the value of the counter.  Since there is
 	 * only one possible increment, the only way to wrap the counter
 	 * is to have a huge number of counter decrements, which requires
 	 * a huge number of tasks and huge SRCU read-side critical-section
@@ -234,8 +234,7 @@ void __srcu_read_unlock(struct srcu_struct *sp, int idx)
 {
 	preempt_disable();
 	smp_mb(); /* C */  /* Avoid leaking the critical section. */
-	ACCESS_ONCE(this_cpu_ptr(sp->per_cpu_ref)->c[idx]) +=
-		SRCU_USAGE_COUNT - 1;
+	ACCESS_ONCE(this_cpu_ptr(sp->per_cpu_ref)->c[idx]) += -1;
 	preempt_enable();
 }
 EXPORT_SYMBOL_GPL(__srcu_read_unlock);
-- 
1.7.4.4
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