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From Davi Arnaut <d...@haxent.com.br>
Subject [patch 1/3] rework win32 condition variable
Date Mon, 30 Apr 2007 20:32:17 GMT
Rework the win32 CV code to signal the condition only if one or more threads
are blocked on the condition variable. If no threads are waiting on the
condition variable, nothing happens.

This patch also eliminates the thundering-herd problem of the manual-reset
event, which (theoretically) wakes up all threads waiting on.

Now the behavior of the CV's should be the same on Unix and win32 platforms.

---
 include/arch/win32/apr_arch_thread_cond.h |    8 -
 locks/win32/thread_cond.c                 |  125 +++++++++++++++++-------------
 2 files changed, 78 insertions(+), 55 deletions(-)

Index: apr/include/arch/win32/apr_arch_thread_cond.h
===================================================================
--- apr.orig/include/arch/win32/apr_arch_thread_cond.h
+++ apr/include/arch/win32/apr_arch_thread_cond.h
@@ -21,10 +21,10 @@
 
 struct apr_thread_cond_t {
     apr_pool_t *pool;
-    HANDLE event;
-    int signal_all;
-    int num_waiting;
-    int signalled;
+    HANDLE semaphore;
+    CRITICAL_SECTION csection;
+    unsigned long num_waiting;
+    unsigned long num_wake;
 };
 
 #endif  /* THREAD_COND_H */
Index: apr/locks/win32/thread_cond.c
===================================================================
--- apr.orig/locks/win32/thread_cond.c
+++ apr/locks/win32/thread_cond.c
@@ -22,59 +22,78 @@
 #include "apr_arch_thread_cond.h"
 #include "apr_portable.h"
 
+#include <limits.h>
+
 static apr_status_t thread_cond_cleanup(void *data)
 {
     apr_thread_cond_t *cond = data;
-    CloseHandle(cond->event);
+    CloseHandle(cond->semaphore);
+    DeleteCriticalSection(&cond->csection);
     return APR_SUCCESS;
 }
 
 APR_DECLARE(apr_status_t) apr_thread_cond_create(apr_thread_cond_t **cond,
                                                  apr_pool_t *pool)
 {
-    *cond = apr_palloc(pool, sizeof(**cond));
-    (*cond)->pool = pool;
-    (*cond)->event = CreateEvent(NULL, TRUE, FALSE, NULL);
-    (*cond)->signal_all = 0;
-    (*cond)->num_waiting = 0;
+    apr_thread_cond_t *cv;
+
+    cv = apr_pcalloc(pool, sizeof(**cond));
+    if (cv == NULL) {
+        return APR_ENOMEM;
+    }
+
+    cv->semaphore = CreateSemaphore(NULL, 0, LONG_MAX, NULL);
+    if (cv->semaphore == NULL) {
+        return apr_get_os_error();
+    }
+
+    *cond = cv;
+    cv->pool = pool;
+    InitializeCriticalSection(&cv->csection);
+    apr_pool_cleanup_register(cv->pool, cv, thread_cond_cleanup,
+                              apr_pool_cleanup_null);
+
     return APR_SUCCESS;
 }
 
+APR_DECLARE(apr_status_t) apr_thread_cond_destroy(apr_thread_cond_t *cond)
+{
+    return apr_pool_cleanup_run(cond->pool, cond, thread_cond_cleanup);
+}
+
 static APR_INLINE apr_status_t _thread_cond_timedwait(apr_thread_cond_t *cond,
                                                       apr_thread_mutex_t *mutex,
                                                       DWORD timeout_ms )
 {
     DWORD res;
+    apr_status_t rv;
 
-    while (1) {
-        cond->num_waiting++;
-
-        apr_thread_mutex_unlock(mutex);
-        res = WaitForSingleObject(cond->event, timeout_ms);
-        apr_thread_mutex_lock(mutex);
-        cond->num_waiting--;
-        if (res != WAIT_OBJECT_0) {
-            apr_status_t rv = apr_get_os_error();
-            if (res == WAIT_TIMEOUT) {
-                return APR_TIMEUP;
-            }
-            return apr_get_os_error();
-        }
-        if (cond->signal_all) {
-            if (cond->num_waiting == 0) {
-                cond->signal_all = 0;
-                cond->signalled = 0;
-                ResetEvent(cond->event);
-            }
+    EnterCriticalSection(&cond->csection);
+    cond->num_waiting++;
+    LeaveCriticalSection(&cond->csection);
+
+    apr_thread_mutex_unlock(mutex);
+
+    do {
+        res = WaitForSingleObject(cond->semaphore, timeout_ms);
+        EnterCriticalSection(&cond->csection);
+        if (cond->num_wake) {
+            cond->num_wake--;
+            rv = APR_SUCCESS;
             break;
         }
-        else if (cond->signalled) {
-            cond->signalled = 0;
-            ResetEvent(cond->event);
+        else if (res != WAIT_OBJECT_0) {
+            cond->num_waiting--;
+            rv = APR_TIMEUP;
             break;
         }
-    }
-    return APR_SUCCESS;
+        LeaveCriticalSection(&cond->csection);
+    } while (1);
+
+    LeaveCriticalSection(&cond->csection);
+    apr_thread_mutex_lock(mutex);
+
+    return rv;
 }
 
 APR_DECLARE(apr_status_t) apr_thread_cond_wait(apr_thread_cond_t *cond,
@@ -94,35 +113,39 @@ APR_DECLARE(apr_status_t) apr_thread_con
 
 APR_DECLARE(apr_status_t) apr_thread_cond_signal(apr_thread_cond_t *cond)
 {
-    apr_status_t rv = APR_SUCCESS;
-    DWORD res;
+    unsigned int wake = 0;
 
-    cond->signalled = 1;
-    res = SetEvent(cond->event);
-    if (res == 0) {
-        rv = apr_get_os_error();
+    EnterCriticalSection(&cond->csection);
+    if (cond->num_waiting) {
+        wake = 1;
+        cond->num_wake++;
+        cond->num_waiting--;
     }
-    return rv;
+    LeaveCriticalSection(&cond->csection);
+
+    if (wake) {
+        ReleaseSemaphore(cond->semaphore, 1, NULL);
+    }
+
+    return APR_SUCCESS;
 }
 
 APR_DECLARE(apr_status_t) apr_thread_cond_broadcast(apr_thread_cond_t *cond)
 {
-    apr_status_t rv = APR_SUCCESS;
-    DWORD res;
+    unsigned long num_wake = 0;
 
-    cond->signalled = 1;
-    cond->signal_all = 1;
-    res = SetEvent(cond->event);
-    if (res == 0) {
-        rv = apr_get_os_error();
+    EnterCriticalSection(&cond->csection);
+    if (cond->num_waiting) {
+        cond->num_wake += num_wake = cond->num_waiting;
+        cond->num_waiting = 0;
     }
-    return rv;
-}
+    LeaveCriticalSection(&cond->csection);
 
-APR_DECLARE(apr_status_t) apr_thread_cond_destroy(apr_thread_cond_t *cond)
-{
-    return apr_pool_cleanup_run(cond->pool, cond, thread_cond_cleanup);
+    if (num_wake) {
+        ReleaseSemaphore(cond->semaphore, num_wake, NULL);
+    }
+
+    return APR_SUCCESS;
 }
 
 APR_POOL_IMPLEMENT_ACCESSOR(thread_cond)
-

--

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