[PATCH] rcu_nest: fix concurrency issues

[Junchang Wang <junchangwang@gmail.com>]

Variable "rcu_gp_ctr" is incremented by the updater and is fetched by
readers concurrently. So protect this variable by using READ_ONCE()
and WRITE_ONCE().

Per-thread variable "rcu_read_gp" is updated by the reader and is read
by the updater. So protect it by using READ_ONCE() and WRITE_ONCE().

The type of "rcu_gp_ctr" is changed to unsigned long because the behavior
of the overflow of a signed long integer is not well defined in C yet.

Refine the code snippet in "rcu_read_lock" that allows a reader to start
over. In this version, we add micro MAX_GP_ADV_DISTANCE which is by
default set to (RCU_GP_CTR_NEST_MASK << 8). Once a reader notices that
MAX_GP_ADV_DISTANCE grace-periods have elapsed since fetching the value of
"rcu_reader_gp", the reader starts over.

 
Signed-off-by: Junchang Wang <junchangwang@gmail.com>
---
 CodeSamples/defer/rcu_nest.c |  5 +++--
 CodeSamples/defer/rcu_nest.h | 25 +++++++++++++++++--------
 2 files changed, 20 insertions(+), 10 deletions(-)

diff --git a/CodeSamples/defer/rcu_nest.c b/CodeSamples/defer/rcu_nest.c
index 64e4087..362f466 100644
--- a/CodeSamples/defer/rcu_nest.c
+++ b/CodeSamples/defer/rcu_nest.c
@@ -35,7 +35,7 @@ void synchronize_rcu(void)
 
 	/* Advance to a new grace-period number, enforce ordering. */
 
-	rcu_gp_ctr += RCU_GP_CTR_BOTTOM_BIT;
+	WRITE_ONCE(rcu_gp_ctr, rcu_gp_ctr + RCU_GP_CTR_BOTTOM_BIT);
 	smp_mb();
 
 	/*
@@ -45,7 +45,8 @@ void synchronize_rcu(void)
 
 	for_each_thread(t) {
 		while (rcu_gp_ongoing(t) &&
-		       ((per_thread(rcu_reader_gp, t) - rcu_gp_ctr) < 0)) {
+		       ((READ_ONCE(per_thread(rcu_reader_gp, t)) -
+			 rcu_gp_ctr) < 0)) {
 			/*@@@ poll(NULL, 0, 10); */
 			barrier();
 		}
diff --git a/CodeSamples/defer/rcu_nest.h b/CodeSamples/defer/rcu_nest.h
index bcc4cde..65ce203 100644
--- a/CodeSamples/defer/rcu_nest.h
+++ b/CodeSamples/defer/rcu_nest.h
@@ -24,8 +24,9 @@ DEFINE_SPINLOCK(rcu_gp_lock);
 #define RCU_GP_CTR_SHIFT 7
 #define RCU_GP_CTR_BOTTOM_BIT (1 << RCU_GP_CTR_SHIFT)
 #define RCU_GP_CTR_NEST_MASK (RCU_GP_CTR_BOTTOM_BIT - 1)
-long rcu_gp_ctr = 0;	/* increment by RCU_GP_CTR_BOTTOM_BIT each gp. */
-DEFINE_PER_THREAD(long, rcu_reader_gp);
+#define MAX_GP_ADV_DISTANCE (RCU_GP_CTR_NEST_MASK << 8)
+unsigned long rcu_gp_ctr = 0;	/* increment by RCU_GP_CTR_BOTTOM_BIT each gp. */
+DEFINE_PER_THREAD(unsigned long, rcu_reader_gp);
 
 static inline int rcu_gp_ongoing(int cpu)
 {
@@ -39,8 +40,8 @@ static void rcu_init(void)
 
 static void rcu_read_lock(void)
 {
-	long tmp;
-	long *rrgp;
+	unsigned long tmp;
+	unsigned long *rrgp;
 
 	/*
 	 * If this is the outermost RCU read-side critical section,
@@ -52,13 +53,21 @@ static void rcu_read_lock(void)
 retry:
 	tmp = *rrgp;
 	if ((tmp & RCU_GP_CTR_NEST_MASK) == 0)
-		tmp = rcu_gp_ctr;
+		tmp = READ_ONCE(rcu_gp_ctr);
 	tmp++;
-	*rrgp = tmp;
+	WRITE_ONCE(*rrgp, tmp);
 	smp_mb();
+
+	/*
+	 * A reader could be suspended in between fetching the value of *rrgp
+	 * and writting the updated value back into *rrgp. During this
+	 * time period, the grace-period counter might have advanced very far.
+	 * In this case, we force the reader to start over.
+	 */
+
 	if (((tmp & RCU_GP_CTR_NEST_MASK) == 1) &&
-	    ((rcu_gp_ctr - tmp) > (RCU_GP_CTR_NEST_MASK << 8)) != 0) {
-		(*rrgp)--;
+	    (READ_ONCE(rcu_gp_ctr) > tmp + MAX_GP_ADV_DISTANCE)) {
+		WRITE_ONCE(*rrgp, *rrgp - 1);
 		goto retry;
 	}
 }
-- 
2.7.4