svn commit: r1067679 - /subversion/trunk/subversion/libsvn_fs_util/caching.c

[st...@apache.org]

Author: stefan2
Date: Sun Feb  6 15:04:03 2011
New Revision: 1067679

URL: http://svn.apache.org/viewvc?rev=1067679&view=rev
Log:
Defer cache initialization to first usage: mod_dav_svn may have to 
call svn_fs_set_cache_config() more than once if the Apache config
contains multiple caching related parameters (not today, but in the
near future). Also, this reduces the memory footprint if not all server
instances actually access FSFS repositories.

* subversion/libsvn_fs_util/caching.c
  (svn_fs__get_global_membuffer_cache): make it thread-safe and 
   allow for concurrent initialization.
  (svn_fs_set_cache_config): don't create caches as a side-effect.

Modified:
    subversion/trunk/subversion/libsvn_fs_util/caching.c

Modified: subversion/trunk/subversion/libsvn_fs_util/caching.c
URL: http://svn.apache.org/viewvc/subversion/trunk/subversion/libsvn_fs_util/caching.c?rev=1067679&r1=1067678&r2=1067679&view=diff
==============================================================================
--- subversion/trunk/subversion/libsvn_fs_util/caching.c (original)
+++ subversion/trunk/subversion/libsvn_fs_util/caching.c Sun Feb  6 15:04:03 2011
@@ -20,6 +20,8 @@
  * ====================================================================
  */
 
+#include <apr_atomic.h>
+
 #include "svn_fs.h"
 #include "private/svn_fs_private.h"
 #include "private/svn_cache.h"
@@ -53,11 +55,14 @@ svn_fs_get_cache_config(void)
 svn_membuffer_t *
 svn_fs__get_global_membuffer_cache(void)
 {
-  static svn_membuffer_t *cache = NULL;
+  static volatile svn_membuffer_t *cache = NULL;
 
   apr_uint64_t cache_size = cache_settings.cache_size;
   if (!cache && cache_size)
     {
+      svn_membuffer_t *old_cache = NULL;
+      svn_membuffer_t *new_cache = NULL;
+
       /* auto-allocate cache*/
       apr_allocator_t *allocator = NULL;
       apr_pool_t *pool = NULL;
@@ -75,23 +80,27 @@ svn_fs__get_global_membuffer_cache(void)
       pool = svn_pool_create_ex(NULL, allocator);
 
       svn_cache__membuffer_cache_create
-          (&cache,
+          (&new_cache,
            (apr_size_t)cache_size,
            (apr_size_t)cache_size / 16,
            ! svn_fs_get_cache_config()->single_threaded,
            pool);
+
+      /* Handle race condition: if we are the first to create a
+       * cache object, make it our global singleton. Otherwise,
+       * discard the new cache and keep the existing one.
+       */
+      old_cache = apr_atomic_casptr(&cache, new_cache, NULL);
+      if (old_cache != NULL)
+        apr_pool_destroy(pool);
     }
 
   return cache;
 }
 
-void 
+void
 svn_fs_set_cache_config(const svn_fs_cache_config_t *settings)
 {
   cache_settings = *settings;
-
-  /* Allocate global membuffer cache as a side-effect.
-   * Only the first call will actually take affect. */
-  svn_fs__get_global_membuffer_cache();
 }