update rwclock to be fair

Update rwlock code from dpdk to be fair when locking/unlocking for all threads. Signed-off-by: Keith Wiles <[email protected]>
CloudNativeDataPlane · Sep 12, 2023 · 5f0a018 · 5f0a018
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Showing 3 changed files with 164 additions and 38 deletions.
diff --git a/lib/include/cne_lock_annotations.h b/lib/include/cne_lock_annotations.h
@@ -0,0 +1,62 @@
+/* SPDX-License-Identifier: BSD-3-Clause
+ * Copyright(c) 2022 Red Hat, Inc.
+ */
+
+#ifndef CNE_LOCK_ANNOTATIONS_H
+#define CNE_LOCK_ANNOTATIONS_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#ifdef CNE_ANNOTATE_LOCKS
+
+#define __cne_lockable __attribute__((lockable))
+
+#define __cne_guarded_by(...) __attribute__((guarded_by(__VA_ARGS__)))
+#define __cne_guarded_var     __attribute__((guarded_var))
+
+#define __cne_exclusive_locks_required(...) __attribute__((exclusive_locks_required(__VA_ARGS__)))
+#define __cne_exclusive_lock_function(...)  __attribute__((exclusive_lock_function(__VA_ARGS__)))
+#define __cne_exclusive_trylock_function(ret, ...) \
+    __attribute__((exclusive_trylock_function(ret, __VA_ARGS__)))
+#define __cne_assert_exclusive_lock(...) __attribute__((assert_exclusive_lock(__VA_ARGS__)))
+
+#define __cne_shared_locks_required(...) __attribute__((shared_locks_required(__VA_ARGS__)))
+#define __cne_shared_lock_function(...)  __attribute__((shared_lock_function(__VA_ARGS__)))
+#define __cne_shared_trylock_function(ret, ...) \
+    __attribute__((shared_trylock_function(ret, __VA_ARGS__)))
+#define __cne_assert_shared_lock(...) __attribute__((assert_shared_lock(__VA_ARGS__)))
+
+#define __cne_unlock_function(...) __attribute__((unlock_function(__VA_ARGS__)))
+
+#define __cne_no_thread_safety_analysis __attribute__((no_thread_safety_analysis))
+
+#else /* ! CNE_ANNOTATE_LOCKS */
+
+#define __cne_lockable
+
+#define __cne_guarded_by(...)
+#define __cne_guarded_var
+
+#define __cne_exclusive_locks_required(...)
+#define __cne_exclusive_lock_function(...)
+#define __cne_exclusive_trylock_function(...)
+#define __cne_assert_exclusive_lock(...)
+
+#define __cne_shared_locks_required(...)
+#define __cne_shared_lock_function(...)
+#define __cne_shared_trylock_function(...)
+#define __cne_assert_shared_lock(...)
+
+#define __cne_unlock_function(...)
+
+#define __cne_no_thread_safety_analysis
+
+#endif /* CNE_ANNOTATE_LOCKS */
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* CNE_LOCK_ANNOTATIONS_H */
diff --git a/lib/include/cne_rwlock.h b/lib/include/cne_rwlock.h
@@ -15,11 +15,17 @@
  * 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/
  */
 
 #include <cne_branch_prediction.h>
 #include <cne_spinlock.h>
 #include <cne_common.h>
+#include <cne_lock_annotations.h>
 #include <cne_pause.h>
 
 #ifdef __cplusplus
@@ -29,10 +35,28 @@ extern "C" {
 /**
  * The cne_rwlock_t type.
  *
- * cnt is -1 when write lock is held, and > 0 when read locks are held.
+ * Readers increment the counter by CNE_RWLOCK_READ (4)
+ * Writers set the CNE_RWLOCK_WRITE bit when lock is held
+ *     and set the CNE_RWLOCK_WAIT bit while waiting.
+ *
+ * 31                 2 1 0
+ * +-------------------+-+-+
+ * |  readers          | | |
+ * +-------------------+-+-+
+ *                      ^ ^
+ *                      | |
+ * WRITE: lock held ----/ |
+ * WAIT: writer pending --/
  */
-typedef struct {
-    volatile int32_t cnt; /**< -1 when W lock held, > 0 when R locks held. */
+
+#define CNE_RWLOCK_WAIT  0x1 /* Writer is waiting */
+#define CNE_RWLOCK_WRITE 0x2 /* Writer has the lock */
+#define CNE_RWLOCK_MASK  (CNE_RWLOCK_WAIT | CNE_RWLOCK_WRITE)
+/* Writer is waiting or has lock */
+#define CNE_RWLOCK_READ 0x4 /* Reader increment */
+
+typedef struct __cne_lockable {
+    int32_t cnt;
 } cne_rwlock_t;
 
 /**
@@ -63,24 +87,29 @@ cne_rwlock_init(cne_rwlock_t *rwl)
  */
 static inline void
 cne_rwlock_read_lock(cne_rwlock_t *rwl)
+    __cne_shared_lock_function(rwl) __cne_no_thread_safety_analysis
 {
     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) & CNE_RWLOCK_MASK)
             cne_pause();
-            continue;
-        }
-        success = __atomic_compare_exchange_n(&rwl->cnt, &x, x + 1, 1, __ATOMIC_ACQUIRE,
-                                              __ATOMIC_RELAXED);
+
+        /* Try to get read lock */
+        x = __atomic_fetch_add(&rwl->cnt, CNE_RWLOCK_READ, __ATOMIC_ACQUIRE) + CNE_RWLOCK_READ;
+
+        /* If no writer, then acquire was successful */
+        if (likely(!(x & CNE_RWLOCK_MASK)))
+            return;
+
+        /* Lost race with writer, backout the change. */
+        __atomic_fetch_sub(&rwl->cnt, CNE_RWLOCK_READ, __ATOMIC_RELAXED);
     }
 }
 
 /**
- * try to take a read lock.
+ * Try to take a read lock.
  *
  * @param rwl
  *   A pointer to a rwlock structure.
@@ -91,19 +120,25 @@ cne_rwlock_read_lock(cne_rwlock_t *rwl)
  */
 static inline int
 cne_rwlock_read_trylock(cne_rwlock_t *rwl)
+    __cne_shared_trylock_function(0, rwl) __cne_no_thread_safety_analysis
 {
     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 & CNE_RWLOCK_MASK)
+        return -EBUSY;
+
+    /* Try to get read lock */
+    x = __atomic_fetch_add(&rwl->cnt, CNE_RWLOCK_READ, __ATOMIC_ACQUIRE) + CNE_RWLOCK_READ;
 
+    /* Back out if writer raced in */
+    if (unlikely(x & CNE_RWLOCK_MASK)) {
+        __atomic_fetch_sub(&rwl->cnt, CNE_RWLOCK_READ, __ATOMIC_RELEASE);
+
+        return -EBUSY;
+    }
     return 0;
 }
 
@@ -114,13 +149,13 @@ cne_rwlock_read_trylock(cne_rwlock_t *rwl)
  *   A pointer to the rwlock structure.
  */
 static inline void
-cne_rwlock_read_unlock(cne_rwlock_t *rwl)
+cne_rwlock_read_unlock(cne_rwlock_t *rwl) __cne_unlock_function(rwl) __cne_no_thread_safety_analysis
 {
-    __atomic_fetch_sub(&rwl->cnt, 1, __ATOMIC_RELEASE);
+    __atomic_fetch_sub(&rwl->cnt, CNE_RWLOCK_READ, __ATOMIC_RELEASE);
 }
 
 /**
- * try to take a write lock.
+ * Try to take a write lock.
  *
  * @param rwl
  *   A pointer to a rwlock structure.
@@ -131,15 +166,16 @@ cne_rwlock_read_unlock(cne_rwlock_t *rwl)
  */
 static inline int
 cne_rwlock_write_trylock(cne_rwlock_t *rwl)
+    __cne_exclusive_trylock_function(0, rwl) __cne_no_thread_safety_analysis
 {
     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 < CNE_RWLOCK_WRITE && __atomic_compare_exchange_n(&rwl->cnt, &x, x + CNE_RWLOCK_WRITE, 1,
+                                                            __ATOMIC_ACQUIRE, __ATOMIC_RELAXED))
+        return 0;
+    else
         return -EBUSY;
-
-    return 0;
 }
 
 /**
@@ -150,19 +186,28 @@ cne_rwlock_write_trylock(cne_rwlock_t *rwl)
  */
 static inline void
 cne_rwlock_write_lock(cne_rwlock_t *rwl)
+    __cne_exclusive_lock_function(rwl) __cne_no_thread_safety_analysis
 {
     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) {
-            cne_pause();
-            continue;
+
+        /* No readers or writers? */
+        if (likely(x < CNE_RWLOCK_WRITE)) {
+            /* Turn off CNE_RWLOCK_WAIT, turn on CNE_RWLOCK_WRITE */
+            if (__atomic_compare_exchange_n(&rwl->cnt, &x, CNE_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 & CNE_RWLOCK_WAIT))
+            __atomic_fetch_or(&rwl->cnt, CNE_RWLOCK_WAIT, __ATOMIC_RELAXED);
+
+        /* Wait until no readers before trying again */
+        while (__atomic_load_n(&rwl->cnt, __ATOMIC_RELAXED) > CNE_RWLOCK_WAIT)
+            cne_pause();
     }
 }
 
@@ -174,8 +219,26 @@ cne_rwlock_write_lock(cne_rwlock_t *rwl)
  */
 static inline void
 cne_rwlock_write_unlock(cne_rwlock_t *rwl)
+    __cne_unlock_function(rwl) __cne_no_thread_safety_analysis
 {
-    __atomic_store_n(&rwl->cnt, 0, __ATOMIC_RELEASE);
+    __atomic_fetch_sub(&rwl->cnt, CNE_RWLOCK_WRITE, __ATOMIC_RELEASE);
+}
+
+/**
+ * Test if the write lock is taken.
+ *
+ * @param rwl
+ *   A pointer to a rwlock structure.
+ * @return
+ *   1 if the write lock is currently taken; 0 otherwise.
+ */
+static inline int
+cne_rwlock_write_is_locked(cne_rwlock_t *rwl)
+{
+    if (__atomic_load_n(&rwl->cnt, __ATOMIC_RELAXED) & CNE_RWLOCK_WRITE)
+        return 1;
+
+    return 0;
 }
 
 /**

diff --git a/lib/include/meson.build b/lib/include/meson.build
@@ -15,6 +15,7 @@ headers = files(
     'cne_inet4.h',
     'cne_inet6.h',
     'cne_isa.h',
+    'cne_lock_annotations.h',
     'cne_lport.h',
     'cne_mutex_helper.h',
     'cne_pause.h',