The original reader/writer lock in DPDK can cause a stream
of readers to starve writers.
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 <step...@networkplumber.org>
---
Would like this to be in 22.11, but needs some more review
lib/eal/include/generic/rte_rwlock.h | 93 ++++++++++++++++++----------
1 file changed, 61 insertions(+), 32 deletions(-)
diff --git a/lib/eal/include/generic/rte_rwlock.h
b/lib/eal/include/generic/rte_rwlock.h
index da9bc3e9c0e2..725cd19ffb27 100644
--- a/lib/eal/include/generic/rte_rwlock.h
+++ b/lib/eal/include/generic/rte_rwlock.h
@@ -13,7 +13,7 @@
* This file defines an API for read-write locks. The lock is used to
* protect data that allows multiple readers in parallel, but only
* one writer. All readers are blocked until the writer is finished
- * writing.
+ * writing. This version will not starve writers.
*
*/
@@ -28,10 +28,17 @@ extern "C" {
/**
* 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 RW_READ (4)
+ * Writers set the RWLOCK_WRITE bit when lock is held
+ * and set the RWLOCK_WAIT bit while waiting.
*/
+
+#define RTE_RWLOCK_WAIT 0x1 /* Writer is waiting */
+#define RTE_RWLOCK_WRITE 0x2 /* Writer has the lock */
+#define RTE_RWLOCK_READ 0x4 /* Reader increment */
+
typedef struct {
- volatile int32_t cnt; /**< -1 when W lock held, > 0 when R locks held.
*/
+ volatile int32_t cnt;
} rte_rwlock_t;
/**
@@ -61,17 +68,24 @@ static inline void
rte_rwlock_read_lock(rte_rwlock_t *rwl)
{
int32_t x;
- int success = 0;
- while (success == 0) {
+ while (1) {
x = __atomic_load_n(&rwl->cnt, __ATOMIC_RELAXED);
/* write lock is held */
- if (x < 0) {
+ if (x & (RTE_RWLOCK_WAIT | RTE_RWLOCK_WRITE)) {
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 (!(x & (RTE_RWLOCK_WAIT | RTE_RWLOCK_WRITE)))
+ return;
+
+ /* Undo */
+ __atomic_fetch_sub(&rwl->cnt, RTE_RWLOCK_READ,
+ __ATOMIC_RELEASE);
}
}
@@ -93,17 +107,23 @@ 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);
+
+ /* write lock is held */
+ if (x & (RTE_RWLOCK_WAIT | RTE_RWLOCK_WRITE))
+ return -EBUSY;
+
+ /* Try to get read lock */
+ x = __atomic_add_fetch(&rwl->cnt, RTE_RWLOCK_READ,
+ __ATOMIC_ACQUIRE);
+
+ if (x & (RTE_RWLOCK_WAIT | RTE_RWLOCK_WRITE)) {
+ __atomic_fetch_sub(&rwl->cnt, RTE_RWLOCK_READ,
+ __ATOMIC_RELEASE);
+ return -EBUSY;
+ }
return 0;
}
@@ -116,7 +136,7 @@ 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 +159,12 @@ 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 +177,26 @@ 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 (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 can try to take the lock */
+ while (__atomic_load_n(&rwl->cnt, __ATOMIC_RELAXED) >
RTE_RWLOCK_WAIT)
+ rte_pause();
+ }
}
/**
@@ -179,7 +208,7 @@ 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
