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DBusAtomic: on Unix, use pthreads mutexes for fallback

Browse source 
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>
This commit is contained in:
Simon McVittie 2013-04-15 13:51:19 +01:00
parent 7ac9b68220
commit c36f21a2e9
4 changed files with 20 additions and 17 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

5
dbus/dbus-internals.h
View file

@ -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);

27
dbus/dbus-sysdeps-unix.c
View file

@ -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
}

2
dbus/dbus-sysdeps-win.c
View file

@ -3188,8 +3188,6 @@ _dbus_get_standard_system_servicedirs (DBusList **dirs)
return TRUE;
}
_DBUS_DEFINE_GLOBAL_LOCK (atomic);
/**
* Atomically increments an integer
*

3
dbus/dbus-threads.c
View file

@ -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),