DBusAtomic: on Unix, use pthreads mutexes for fallback

On pthreads platforms, POSIX guarantees that we can "allocate" mutexes
as library-global variables, without involving malloc. This means we
don't need to error-check their allocation - if the dynamic linker
succeeds, then we have enough memory for all our globals - which is an
important step towards being thread-safe by default. In particular,
making atomic operations never rely on DBusMutex means that we are free
to implement parts of DBusMutex in terms of DBusAtomic, if it would help.

We do not currently support any non-Windows platform that does not have
pthreads. This is unlikely to change.

On Windows, we already used real atomic operations; we can just
delete the unused global variable.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=54972
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Reviewed-by: Alban Crequy <alban.crequy@collabora.co.uk>
Reviewed-by: Ralf Habacker <ralf.habacker@freenet.de>

diff --git a/dbus/dbus-internals.h b/dbus/dbus-internals.h
index 80376ad..28486a8 100644 (file)
--- a/dbus/dbus-internals.h
+++ b/dbus/dbus-internals.h
@@ -323,12 +323,7 @@ _DBUS_DECLARE_GLOBAL_LOCK (win_fds);
 _DBUS_DECLARE_GLOBAL_LOCK (sid_atom_cache);
 _DBUS_DECLARE_GLOBAL_LOCK (machine_uuid);
 
-#if !DBUS_USE_SYNC
-_DBUS_DECLARE_GLOBAL_LOCK (atomic);
-#define _DBUS_N_GLOBAL_LOCKS (15)
-#else
 #define _DBUS_N_GLOBAL_LOCKS (14)
-#endif
 
 dbus_bool_t _dbus_threads_init_debug (void);
 

diff --git a/dbus/dbus-sysdeps-unix.c b/dbus/dbus-sysdeps-unix.c
index 88db415..34dfbe7 100644 (file)
--- a/dbus/dbus-sysdeps-unix.c
+++ b/dbus/dbus-sysdeps-unix.c
@@ -82,6 +82,10 @@
 
 #include "sd-daemon.h"
 
+#if !DBUS_USE_SYNC
+#include <pthread.h>
+#endif
+
 #ifndef O_BINARY
 #define O_BINARY 0
 #endif
@@ -2428,7 +2432,12 @@ _dbus_parse_uid (const DBusString      *uid_str,
 }
 
 #if !DBUS_USE_SYNC
-_DBUS_DEFINE_GLOBAL_LOCK (atomic);
+/* To be thread-safe by default on platforms that don't necessarily have
+ * atomic operations (notably Debian armel, which is armv4t), we must
+ * use a mutex that can be initialized statically, like this.
+ * GLib >= 2.32 uses a similar system.
+ */
+static pthread_mutex_t atomic_mutex = PTHREAD_MUTEX_INITIALIZER;
 #endif
 
 /**
@@ -2444,10 +2453,12 @@ _dbus_atomic_inc (DBusAtomic *atomic)
   return __sync_add_and_fetch(&atomic->value, 1)-1;
 #else
   dbus_int32_t res;
-  _DBUS_LOCK (atomic);
+
+  pthread_mutex_lock (&atomic_mutex);
   res = atomic->value;
   atomic->value += 1;
-  _DBUS_UNLOCK (atomic);
+  pthread_mutex_unlock (&atomic_mutex);
+
   return res;
 #endif
 }
@@ -2466,10 +2477,11 @@ _dbus_atomic_dec (DBusAtomic *atomic)
 #else
   dbus_int32_t res;
 
-  _DBUS_LOCK (atomic);
+  pthread_mutex_lock (&atomic_mutex);
   res = atomic->value;
   atomic->value -= 1;
-  _DBUS_UNLOCK (atomic);
+  pthread_mutex_unlock (&atomic_mutex);
+
   return res;
 #endif
 }
@@ -2490,9 +2502,10 @@ _dbus_atomic_get (DBusAtomic *atomic)
 #else
   dbus_int32_t res;
 
-  _DBUS_LOCK (atomic);
+  pthread_mutex_lock (&atomic_mutex);
   res = atomic->value;
-  _DBUS_UNLOCK (atomic);
+  pthread_mutex_unlock (&atomic_mutex);
+
   return res;
 #endif
 }

diff --git a/dbus/dbus-sysdeps-win.c b/dbus/dbus-sysdeps-win.c
index 66d6b76..4a54981 100644 (file)
--- a/dbus/dbus-sysdeps-win.c
+++ b/dbus/dbus-sysdeps-win.c
@@ -3188,8 +3188,6 @@ _dbus_get_standard_system_servicedirs (DBusList **dirs)
   return TRUE;
 }
 
-_DBUS_DEFINE_GLOBAL_LOCK (atomic);
-
 /**
  * Atomically increments an integer
  *

diff --git a/dbus/dbus-threads.c b/dbus/dbus-threads.c
index bb1169d..b464629 100644 (file)
--- a/dbus/dbus-threads.c
+++ b/dbus/dbus-threads.c
@@ -496,9 +496,6 @@ init_locks (void)
     LOCK_ADDR (pending_call_slots),
     LOCK_ADDR (server_slots),
     LOCK_ADDR (message_slots),
-#if !DBUS_USE_SYNC
-    LOCK_ADDR (atomic),
-#endif
     LOCK_ADDR (bus),
     LOCK_ADDR (bus_datas),
     LOCK_ADDR (shutdown_funcs),