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 80376ad52ba1f69904cd50dfc753c6d2359b0bbd..28486a81aa1b692cd50a03e7c1c6578509f4da37 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 88db41537d112287f370017b3234c65ac9aa76b8..34dfbe7d3eecebc7e06e1ee0c369f4089e385375 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 66d6b76759ed2fd4e9fe9bcbf16f232d5c528834..4a5498170baaf5625c2053d00f84d8a7228c73cc 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 bb1169db0f3bf7e265ee2de8b00a0949524f7af5..b464629a062fe0ebd79141ede6fbd7002c752fe0 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),