From bda4d6e0d01116db59a0a03b0c703a7af6e11949 Mon Sep 17 00:00:00 2001
From: Rob Clark <robdclark@chromium.org>
Date: Mon, 16 Nov 2020 10:54:20 -0800
Subject: [PATCH] util: Add helpers for various one-time-init patters

A fairly common pattern for debug envvars is something like:

   static int should_print = -1;
   if (should_print < 0)
      should_print = env_var_as_unsigned("NIR_PRINT", 0);

Unfortunately helgrind doesn't realize that we expect to always get the
same return value, so we don't actually care about the race condition
here.

Add a helper get_once() and do_once macros, with extra locking to make
helgrind/drd happy.  Note that other than the nir usages (which are
limited to debug builds), other usages are not in hot-paths.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7644>
---
 src/util/debug.h   | 61 ++++++++++++++++++++++++++++++++++++++++++++++
 src/util/u_debug.h | 33 ++++---------------------
 2 files changed, 66 insertions(+), 28 deletions(-)

diff --git a/src/util/debug.h b/src/util/debug.h
index bbcc1975545..e6a53e2fdec 100644
--- a/src/util/debug.h
+++ b/src/util/debug.h
@@ -27,6 +27,8 @@
 #include <stdint.h>
 #include <stdbool.h>
 
+#include "util/simple_mtx.h"
+
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -50,4 +52,63 @@ env_var_as_unsigned(const char *var_name, unsigned default_value);
 } /* extern C */
 #endif
 
+/**
+ * Helper for arbitrary one-time initialization, with additional locking
+ * to ensure the initialization only happens once (and to make tools like
+ * helgrind happy).
+ */
+#define do_once for( \
+      struct __do_once_data *__d = ({                 \
+         static struct __do_once_data __sd = {        \
+               .lock = _SIMPLE_MTX_INITIALIZER_NP,    \
+               .done = false,                         \
+         };                                           \
+         simple_mtx_lock(&__sd.lock);                 \
+         &__sd;                                       \
+      });                                             \
+      ({                                              \
+         if (__d->done)                               \
+            simple_mtx_unlock(&__d->lock);            \
+         !__d->done;                                  \
+      });                                             \
+      __d->done = true)
+
+struct __do_once_data {
+   simple_mtx_t lock;
+   bool done;
+};
+
+/**
+ * Helper for one-time debug value from env-var, and other similar cases,
+ * where the expression is expected to return the same value each time.
+ *
+ * This has additional locking, compared to open-coding the initialization,
+ * to make tools like helgrind happy.
+ */
+#define get_once(__expr) ({                           \
+      static __typeof__(__expr) __val;                \
+      do_once {                                       \
+         __val = __expr;                              \
+      }                                               \
+      __val;                                          \
+   })
+
+/**
+ * Alternative version of get_once() which has no locking in release builds,
+ * suitable for hot-paths.
+ */
+#ifndef NDEBUG
+#define get_once_nolock(__expr) ({                    \
+      static bool __once;                             \
+      static __typeof__(__expr) __val;                \
+      if (!__once) {                                  \
+         __val = __expr;                              \
+         __once = true;                               \
+      }                                               \
+      __val;                                          \
+   })
+#else
+#define get_once_nolock(__expr) get_once(__expr)
+#endif
+
 #endif /* _UTIL_DEBUG_H */
diff --git a/src/util/u_debug.h b/src/util/u_debug.h
index fc06c582b0d..d4f77c61251 100644
--- a/src/util/u_debug.h
+++ b/src/util/u_debug.h
@@ -40,6 +40,7 @@
 
 #include <stdarg.h>
 #include <string.h>
+#include "util/debug.h"
 #include "util/os_misc.h"
 #include "util/detect_os.h"
 #include "util/macros.h"
@@ -407,52 +408,28 @@ debug_get_flags_option(const char *name,
 static const char * \
 debug_get_option_ ## suffix (void) \
 { \
-   static bool first = true; \
-   static const char * value; \
-   if (first) { \
-      first = false; \
-      value = debug_get_option(name, dfault); \
-   } \
-   return value; \
+   return get_once_nolock(debug_get_option(name, dfault)); \
 }
 
 #define DEBUG_GET_ONCE_BOOL_OPTION(sufix, name, dfault) \
 static bool \
 debug_get_option_ ## sufix (void) \
 { \
-   static bool first = true; \
-   static bool value; \
-   if (first) { \
-      first = false; \
-      value = debug_get_bool_option(name, dfault); \
-   } \
-   return value; \
+   return get_once_nolock(debug_get_bool_option(name, dfault)); \
 }
 
 #define DEBUG_GET_ONCE_NUM_OPTION(sufix, name, dfault) \
 static long \
 debug_get_option_ ## sufix (void) \
 { \
-   static bool first = true; \
-   static long value; \
-   if (first) { \
-      first = false; \
-      value = debug_get_num_option(name, dfault); \
-   } \
-   return value; \
+   return get_once_nolock(debug_get_num_option(name, dfault)); \
 }
 
 #define DEBUG_GET_ONCE_FLAGS_OPTION(sufix, name, flags, dfault) \
 static unsigned long \
 debug_get_option_ ## sufix (void) \
 { \
-   static bool first = true; \
-   static unsigned long value; \
-   if (first) { \
-      first = false; \
-      value = debug_get_flags_option(name, flags, dfault); \
-   } \
-   return value; \
+   return get_once_nolock(debug_get_flags_option(name, flags, dfault)); \
 }