Fix deadlock in cairo-scaled-font.c

When cairo_scaled_glyph_page_cache needs to remove entries,
cairo-cache calls _cairo_hash_table_random_entry() with the predicate
_cairo_scaled_glyph_page_can_remove(). This function checks that the
glyph_page scaled_font is not locked by testing
scaled_font->cache_frozen. The scaled font is locked in the
cache-cache destroy entry callback: _cairo_scaled_glyph_page_pluck().

There is a race condition here between testing
scaled_font->cache_frozen and locking the font. Fix this by adding a
new CAIRO_MUTEX_TRY_LOCK mutex operation, and using it to test and
lock the scaled font in _cairo_scaled_glyph_page_can_remove().

Fixes the multithreaded case in #440

1 files changed, 6 insertions, 0 deletions

diff --git a/src/cairo-mutex-impl-private.h b/src/cairo-mutex-impl-private.h
index 9f208aaa9..a8599d47e 100644
--- a/src/cairo-mutex-impl-private.h
+++ b/src/cairo-mutex-impl-private.h
@@ -87,6 +87,9 @@
  *   No trailing semicolons are needed (in any macro you define here).
  *   You should be able to compile the following snippet:
  *
+ * - #define CAIRO_MUTEX_IMPL_TRY_LOCK(mutex) to try locking the mutex object,
+ *   returning TRUE if the lock is acquired, FALSE if the mutex could not be locked.
+ *
  *   <programlisting>
  *	cairo_mutex_impl_t _cairo_some_mutex;
  *
@@ -163,6 +166,7 @@
 # define CAIRO_MUTEX_IMPL_NO 1
 # define CAIRO_MUTEX_IMPL_INITIALIZE() CAIRO_MUTEX_IMPL_NOOP
 # define CAIRO_MUTEX_IMPL_LOCK(mutex) CAIRO_MUTEX_IMPL_NOOP1(mutex)
+# define CAIRO_MUTEX_IMPL_TRY_LOCK(mutex) (CAIRO_MUTEX_IMPL_NOOP1(mutex), TRUE)
 # define CAIRO_MUTEX_IMPL_UNLOCK(mutex) CAIRO_MUTEX_IMPL_NOOP1(mutex)
 # define CAIRO_MUTEX_IMPL_NIL_INITIALIZER 0
 
@@ -190,6 +194,7 @@
 
 # define CAIRO_MUTEX_IMPL_WIN32 1
 # define CAIRO_MUTEX_IMPL_LOCK(mutex) EnterCriticalSection (&(mutex))
+# define CAIRO_MUTEX_IMPL_TRY_LOCK(mutex) TryEnterCriticalSection (&(mutex))
 # define CAIRO_MUTEX_IMPL_UNLOCK(mutex) LeaveCriticalSection (&(mutex))
 # define CAIRO_MUTEX_IMPL_INIT(mutex) InitializeCriticalSection (&(mutex))
 # define CAIRO_MUTEX_IMPL_FINI(mutex) DeleteCriticalSection (&(mutex))
@@ -208,6 +213,7 @@
 # define CAIRO_MUTEX_IMPL_INIT(mutex) pthread_mutex_init (&(mutex), NULL)
 #endif
 # define CAIRO_MUTEX_IMPL_LOCK(mutex) pthread_mutex_lock (&(mutex))
+# define CAIRO_MUTEX_IMPL_TRY_LOCK(mutex) (pthread_mutex_trylock (&(mutex)) == 0)
 # define CAIRO_MUTEX_IMPL_UNLOCK(mutex) pthread_mutex_unlock (&(mutex))
 #if HAVE_LOCKDEP
 # define CAIRO_MUTEX_IS_LOCKED(mutex) LOCKDEP_IS_LOCKED (&(mutex))