[PATCH v2] rwlock: prevent readers from starving writers

[Stephen Hemminger]

Modify reader/writer lock to avoid starvation of writer.  The previous
implementation would cause a writer to get starved if readers kept
acquiring the lock.  The new version uses an additional bit to indicate
that a writer is waiting and which keeps readers from starving the
writer.

Signed-off-by: Stephen Hemminger <stephen at networkplumber.org>
Acked-by: Morten Brørup <mb at smartsharesystems.com>
---
v2 - incorporate feedback, change from RFC to PATCH

 lib/eal/include/generic/rte_rwlock.h | 119 +++++++++++++++++++--------
 1 file changed, 83 insertions(+), 36 deletions(-)

diff --git a/lib/eal/include/generic/rte_rwlock.h b/lib/eal/include/generic/rte_rwlock.h
index da9bc3e9c0e2..59ec54110444 100644
--- a/lib/eal/include/generic/rte_rwlock.h
+++ b/lib/eal/include/generic/rte_rwlock.h
@@ -15,23 +15,46 @@
  * one writer. All readers are blocked until the writer is finished
  * writing.
  *
+ * This version does not give preference to readers or writers
+ * and does not starve either readers or writers.
+ *
+ * See also:
+ *  https://locklessinc.com/articles/locks/
  */
 
 #ifdef __cplusplus
 extern "C" {
 #endif
 
+#include <rte_branch_prediction.h>
 #include <rte_common.h>
-#include <rte_atomic.h>
 #include <rte_pause.h>
 
 /**
  * The rte_rwlock_t type.
  *
- * cnt is -1 when write lock is held, and > 0 when read locks are held.
+ * Readers increment the counter by RTE_RWLOCK_READ (4)
+ * Writers set the RTE_RWLOCK_WRITE bit when lock is held
+ *     and set the RTE_RWLOCK_WAIT bit while waiting.
+ *
+ * 31                 2 1 0
+ * +-------------------+-+-+
+ * |  readers          | | |
+ * +-------------------+-+-+
+ *                      ^ ^
+ *                      | |
+ * WRITE: lock held ----/ |
+ * WAIT: writer pending --/
  */
+
+#define RTE_RWLOCK_WAIT	 0x1	/* Writer is waiting */
+#define RTE_RWLOCK_WRITE 0x2	/* Writer has the lock */
+#define RTE_RWLOCK_MASK  (RTE_RWLOCK_WAIT | RTE_RWLOCK_WRITE)
+				/* Writer is waiting or has lock */
+#define RTE_RWLOCK_READ	 0x4	/* Reader increment */
+
 typedef struct {
-	volatile int32_t cnt; /**< -1 when W lock held, > 0 when R locks held. */
+	int32_t cnt;
 } rte_rwlock_t;
 
 /**
@@ -61,17 +84,24 @@ static inline void
 rte_rwlock_read_lock(rte_rwlock_t *rwl)
 {
 	int32_t x;
-	int success = 0;
 
-	while (success == 0) {
-		x = __atomic_load_n(&rwl->cnt, __ATOMIC_RELAXED);
-		/* write lock is held */
-		if (x < 0) {
+	while (1) {
+		/* Wait while writer is present or pending */
+		while (__atomic_load_n(&rwl->cnt, __ATOMIC_RELAXED)
+		       & RTE_RWLOCK_MASK)
 			rte_pause();
-			continue;
-		}
-		success = __atomic_compare_exchange_n(&rwl->cnt, &x, x + 1, 1,
-					__ATOMIC_ACQUIRE, __ATOMIC_RELAXED);
+
+		/* Try to get read lock */
+		x = __atomic_add_fetch(&rwl->cnt, RTE_RWLOCK_READ,
+				       __ATOMIC_ACQUIRE);
+
+		/* If no writer, then acquire was successful */
+		if (likely(!(x & RTE_RWLOCK_MASK)))
+			return;
+
+		/* Lost race with writer, backout the change. */
+		__atomic_fetch_sub(&rwl->cnt, RTE_RWLOCK_READ,
+				   __ATOMIC_RELAXED);
 	}
 }
 
@@ -93,17 +123,24 @@ static inline int
 rte_rwlock_read_trylock(rte_rwlock_t *rwl)
 {
 	int32_t x;
-	int success = 0;
 
-	while (success == 0) {
-		x = __atomic_load_n(&rwl->cnt, __ATOMIC_RELAXED);
-		/* write lock is held */
-		if (x < 0)
-			return -EBUSY;
-		success = __atomic_compare_exchange_n(&rwl->cnt, &x, x + 1, 1,
-					__ATOMIC_ACQUIRE, __ATOMIC_RELAXED);
-	}
+	x = __atomic_load_n(&rwl->cnt, __ATOMIC_RELAXED);
+
+	/* fail if write lock is held or writer is pending */
+	if (x & RTE_RWLOCK_MASK)
+		return -EBUSY;
 
+	/* Try to get read lock */
+	x = __atomic_add_fetch(&rwl->cnt, RTE_RWLOCK_READ,
+			       __ATOMIC_ACQUIRE);
+
+	/* Back out if writer raced in */
+	if (unlikely(x & RTE_RWLOCK_MASK)) {
+		__atomic_fetch_sub(&rwl->cnt, RTE_RWLOCK_READ,
+				   __ATOMIC_RELEASE);
+
+		return -EBUSY;
+	}
 	return 0;
 }
 
@@ -116,7 +153,7 @@ rte_rwlock_read_trylock(rte_rwlock_t *rwl)
 static inline void
 rte_rwlock_read_unlock(rte_rwlock_t *rwl)
 {
-	__atomic_fetch_sub(&rwl->cnt, 1, __ATOMIC_RELEASE);
+	__atomic_fetch_sub(&rwl->cnt, RTE_RWLOCK_READ, __ATOMIC_RELEASE);
 }
 
 /**
@@ -139,11 +176,12 @@ rte_rwlock_write_trylock(rte_rwlock_t *rwl)
 	int32_t x;
 
 	x = __atomic_load_n(&rwl->cnt, __ATOMIC_RELAXED);
-	if (x != 0 || __atomic_compare_exchange_n(&rwl->cnt, &x, -1, 1,
-			      __ATOMIC_ACQUIRE, __ATOMIC_RELAXED) == 0)
+	if (x < RTE_RWLOCK_WRITE &&
+	    __atomic_compare_exchange_n(&rwl->cnt, &x, x + RTE_RWLOCK_WRITE,
+					1, __ATOMIC_ACQUIRE, __ATOMIC_RELAXED))
+		return 0;
+	else
 		return -EBUSY;
-
-	return 0;
 }
 
 /**
@@ -156,18 +194,27 @@ static inline void
 rte_rwlock_write_lock(rte_rwlock_t *rwl)
 {
 	int32_t x;
-	int success = 0;
 
-	while (success == 0) {
+	while (1) {
 		x = __atomic_load_n(&rwl->cnt, __ATOMIC_RELAXED);
-		/* a lock is held */
-		if (x != 0) {
-			rte_pause();
-			continue;
+
+		/* No readers or writers? */
+		if (likely(x < RTE_RWLOCK_WRITE)) {
+			/* Turn off RTE_RWLOCK_WAIT, turn on RTE_RWLOCK_WRITE */
+			if (__atomic_compare_exchange_n(&rwl->cnt, &x, RTE_RWLOCK_WRITE, 1,
+							__ATOMIC_ACQUIRE, __ATOMIC_RELAXED))
+				return;
 		}
-		success = __atomic_compare_exchange_n(&rwl->cnt, &x, -1, 1,
-					__ATOMIC_ACQUIRE, __ATOMIC_RELAXED);
-	}
+
+		/* Turn on writer wait bit */
+		if (!(x & RTE_RWLOCK_WAIT))
+			__atomic_fetch_or(&rwl->cnt, RTE_RWLOCK_WAIT, __ATOMIC_RELAXED);
+
+		/* Wait until no readers befor trying again */
+		while (__atomic_load_n(&rwl->cnt, __ATOMIC_RELAXED) > RTE_RWLOCK_WAIT)
+			rte_pause();
+
+    }
 }
 
 /**
@@ -179,7 +226,7 @@ rte_rwlock_write_lock(rte_rwlock_t *rwl)
 static inline void
 rte_rwlock_write_unlock(rte_rwlock_t *rwl)
 {
-	__atomic_store_n(&rwl->cnt, 0, __ATOMIC_RELEASE);
+	__atomic_fetch_sub(&rwl->cnt, RTE_RWLOCK_WRITE, __ATOMIC_RELEASE);
 }
 
 /**
-- 
2.35.1