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 <step...@networkplumber.org> Acked-by: Morten Brørup <m...@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
