bus: Add a fast-path if log messages don't have to be logged

For some rwlock_t implementations acquiring the read lock could be quite
expensive even if there are no writers (e.g. because the implementation
requires acquiring a mutex to check for writers) particularly if the
lock is highly contended, like it is for the vlog() method.

diff --git a/src/libcharon/bus/bus.c b/src/libcharon/bus/bus.c
index b46184809f854e0ceea98288e8a6758e052d88b9..bc080d1c0b47cebd543e7eca99b14384d8b5dd4c 100644 (file)
--- a/src/libcharon/bus/bus.c
+++ b/src/libcharon/bus/bus.c
@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2011-2012 Tobias Brunner
+ * Copyright (C) 2011-2014 Tobias Brunner
  * Copyright (C) 2006 Martin Willi
  * Hochschule fuer Technik Rapperswil
  *
@@ -23,6 +23,31 @@
 #include <threading/mutex.h>
 #include <threading/rwlock.h>
 
+/**
+ * These operations allow us to speed up the log level checks on some platforms.
+ * In particular if acquiring the read lock is expensive even in the absence of
+ * any writers.
+ *
+ * Note that while holding the read/write lock the read does not have to be
+ * atomic as the write lock must be held to set the level.
+ */
+#ifdef HAVE_GCC_ATOMIC_OPERATIONS
+
+#define skip_level(ptr, level) (__atomic_load_n(ptr, __ATOMIC_RELAXED) < level)
+#define set_level(ptr, val) __atomic_store_n(ptr, val, __ATOMIC_RELAXED)
+
+#elif defined(HAVE_GCC_SYNC_OPERATIONS)
+
+#define skip_level(ptr, level) (__sync_fetch_and_add(ptr, 0) < level)
+#define set_level(ptr, val) __sync_bool_compare_and_swap(ptr, *ptr, val)
+
+#else
+
+#define skip_level(ptr, level) FALSE
+#define set_level(ptr, val) ({ *ptr = val; })
+
+#endif
+
 typedef struct private_bus_t private_bus_t;
 
 /**
@@ -173,11 +198,12 @@ static inline void register_logger(private_bus_t *this, debug_t group,
 
        if (entry->logger->log)
        {
-               this->max_level[group] = max(this->max_level[group], level);
+               set_level(&this->max_level[group], max(this->max_level[group], level));
        }
        if (entry->logger->vlog)
        {
-               this->max_vlevel[group] = max(this->max_vlevel[group], level);
+               set_level(&this->max_vlevel[group],
+                                 max(this->max_vlevel[group], level));
        }
 }
 
@@ -205,6 +231,7 @@ static inline void unregister_logger(private_bus_t *this, logger_t *logger)
 
        if (found)
        {
+               level_t level = LEVEL_SILENT, vlevel = LEVEL_SILENT;
                debug_t group;
 
                for (group = 0; group < DBG_MAX; group++)
@@ -214,13 +241,19 @@ static inline void unregister_logger(private_bus_t *this, logger_t *logger)
                                loggers = this->loggers[group];
                                loggers->remove(loggers, found, NULL);
 
-                               this->max_level[group] = LEVEL_SILENT;
-                               this->max_vlevel[group] = LEVEL_SILENT;
                                if (loggers->get_first(loggers, (void**)&entry) == SUCCESS)
                                {
-                                       this->max_level[group] = entry->levels[group];
-                                       this->max_vlevel[group] = entry->levels[group];
+                                       if (entry->logger->log)
+                                       {
+                                               level = entry->levels[group];
+                                       }
+                                       if (entry->logger->vlog)
+                                       {
+                                               vlevel = entry->levels[group];
+                                       }
                                }
+                               set_level(&this->max_level[group], level);
+                               set_level(&this->max_vlevel[group], vlevel);
                        }
                }
                free(found);
@@ -324,6 +357,19 @@ METHOD(bus_t, vlog, void,
        linked_list_t *loggers;
        log_data_t data;
 
+       /* NOTE: This is not 100% thread-safe and done here only because it is
+        * performance critical.  We therefore ignore the following two issues for
+        * this particular case:  1) We might miss some log messages if another
+        * thread concurrently increases the log level or registers a new logger.
+        * 2) We might have to acquire the read lock below even if it wouldn't be
+        * necessary anymore due to another thread concurrently unregistering a
+        * logger or reducing the level. */
+       if (skip_level(&this->max_level[group], level) &&
+               skip_level(&this->max_vlevel[group], level))
+       {
+               return;
+       }
+
        this->log_lock->read_lock(this->log_lock);
        loggers = this->loggers[group];