// Has this thread written a thread end event to `buffer`?
gboolean ended;
+
+ // Stored in `buffer_lock_state` to take the exclusive lock.
+ int small_id;
} MonoProfilerThread;
+// Do not use these TLS macros directly unless you know what you're doing.
+
+#ifdef HOST_WIN32
+
+#define PROF_TLS_SET(VAL) (TlsSetValue (profiler_tls, (VAL)))
+#define PROF_TLS_GET() ((MonoProfilerThread *) TlsGetValue (profiler_tls))
+#define PROF_TLS_INIT() (profiler_tls = TlsAlloc ())
+#define PROF_TLS_FREE() (TlsFree (profiler_tls))
+
+static DWORD profiler_tls;
+
+#elif HAVE_KW_THREAD
+
+#define PROF_TLS_SET(VAL) (profiler_tls = (VAL))
+#define PROF_TLS_GET() (profiler_tls)
+#define PROF_TLS_INIT()
+#define PROF_TLS_FREE()
+
+static __thread MonoProfilerThread *profiler_tls;
+
+#else
+
+#define PROF_TLS_SET(VAL) (pthread_setspecific (profiler_tls, (VAL)))
+#define PROF_TLS_GET() ((MonoProfilerThread *) pthread_getspecific (profiler_tls))
+#define PROF_TLS_INIT() (pthread_key_create (&profiler_tls, NULL))
+#define PROF_TLS_FREE() (pthread_key_delete (profiler_tls))
+
+static pthread_key_t profiler_tls;
+
+#endif
+
static uintptr_t
thread_id (void)
{
#define EXIT_LOG EXIT_LOG_EXPLICIT (DO_SEND)
-static volatile gint32 buffer_rwlock_count;
-static volatile gpointer buffer_rwlock_exclusive;
-
-// Can be used recursively.
-static void
-buffer_lock (void)
-{
- /*
- * If the thread holding the exclusive lock tries to modify the
- * reader count, just make it a no-op. This way, we also avoid
- * invoking the GC safe point macros below, which could break if
- * done from a thread that is currently the initiator of STW.
- *
- * In other words, we rely on the fact that the GC thread takes
- * the exclusive lock in the gc_event () callback when the world
- * is about to stop.
- */
- if (InterlockedReadPointer (&buffer_rwlock_exclusive) != (gpointer) thread_id ()) {
- MONO_ENTER_GC_SAFE;
-
- while (InterlockedReadPointer (&buffer_rwlock_exclusive))
- mono_thread_info_yield ();
-
- InterlockedIncrement (&buffer_rwlock_count);
-
- MONO_EXIT_GC_SAFE;
- }
-
- mono_memory_barrier ();
-}
-
-static void
-buffer_unlock (void)
-{
- mono_memory_barrier ();
-
- // See the comment in buffer_lock ().
- if (InterlockedReadPointer (&buffer_rwlock_exclusive) == (gpointer) thread_id ())
- return;
-
- g_assert (InterlockedRead (&buffer_rwlock_count) && "Why are we trying to decrement a zero reader count?");
-
- InterlockedDecrement (&buffer_rwlock_count);
-}
-
-// Cannot be used recursively.
-static void
-buffer_lock_excl (void)
-{
- gpointer tid = (gpointer) thread_id ();
-
- g_assert (InterlockedReadPointer (&buffer_rwlock_exclusive) != tid && "Why are we taking the exclusive lock twice?");
-
- MONO_ENTER_GC_SAFE;
-
- while (InterlockedCompareExchangePointer (&buffer_rwlock_exclusive, tid, 0))
- mono_thread_info_yield ();
-
- while (InterlockedRead (&buffer_rwlock_count))
- mono_thread_info_yield ();
-
- MONO_EXIT_GC_SAFE;
-
- mono_memory_barrier ();
-}
-
-static void
-buffer_unlock_excl (void)
-{
- mono_memory_barrier ();
-
- g_assert (InterlockedReadPointer (&buffer_rwlock_exclusive) && "Why is the exclusive lock not held?");
- g_assert (InterlockedReadPointer (&buffer_rwlock_exclusive) == (gpointer) thread_id () && "Why does another thread hold the exclusive lock?");
- g_assert (!InterlockedRead (&buffer_rwlock_count) && "Why are there readers when the exclusive lock is held?");
-
- InterlockedWritePointer (&buffer_rwlock_exclusive, NULL);
-}
-
typedef struct _BinaryObject BinaryObject;
struct _BinaryObject {
BinaryObject *next;
uint64_t time;
} MethodInfo;
-// Do not use these TLS macros directly unless you know what you're doing.
-
-#ifdef HOST_WIN32
-
-#define PROF_TLS_SET(VAL) (TlsSetValue (profiler_tls, (VAL)))
-#define PROF_TLS_GET() ((MonoProfilerThread *) TlsGetValue (profiler_tls))
-#define PROF_TLS_INIT() (profiler_tls = TlsAlloc ())
-#define PROF_TLS_FREE() (TlsFree (profiler_tls))
-
-static DWORD profiler_tls;
-
-#elif HAVE_KW_THREAD
-
-#define PROF_TLS_SET(VAL) (profiler_tls = (VAL))
-#define PROF_TLS_GET() (profiler_tls)
-#define PROF_TLS_INIT()
-#define PROF_TLS_FREE()
-
-static __thread MonoProfilerThread *profiler_tls;
-
-#else
-
-#define PROF_TLS_SET(VAL) (pthread_setspecific (profiler_tls, (VAL)))
-#define PROF_TLS_GET() ((MonoProfilerThread *) pthread_getspecific (profiler_tls))
-#define PROF_TLS_INIT() (pthread_key_create (&profiler_tls, NULL))
-#define PROF_TLS_FREE() (pthread_key_delete (profiler_tls))
-
-static pthread_key_t profiler_tls;
-
-#endif
-
static char*
pstrdup (const char *s)
{
init_buffer_state (thread);
+ thread->small_id = mono_thread_info_register_small_id ();
+
/*
* Some internal profiler threads don't need to be cleaned up
* by the main thread on shutdown.
return new_;
}
+/*
+ * This is a reader/writer spin lock of sorts used to protect log buffers.
+ * When a thread modifies its own log buffer, it increments the reader
+ * count. When a thread wants to access log buffers of other threads, it
+ * takes the exclusive lock.
+ *
+ * `buffer_lock_state` holds the reader count in its lower 16 bits, and
+ * the small ID of the thread currently holding the exclusive (writer)
+ * lock in its upper 16 bits. Both can be zero. It's important that the
+ * whole lock state is a single word that can be read/written atomically
+ * to avoid race conditions where there could end up being readers while
+ * the writer lock is held.
+ *
+ * The lock is writer-biased. When a thread wants to take the exclusive
+ * lock, it increments `buffer_lock_exclusive_intent` which will make new
+ * readers spin until it's back to zero, then takes the exclusive lock
+ * once the reader count has reached zero. After releasing the exclusive
+ * lock, it decrements `buffer_lock_exclusive_intent`, which, when it
+ * reaches zero again, allows readers to increment the reader count.
+ *
+ * The writer bias is necessary because we take the exclusive lock in
+ * `gc_event ()` during STW. If the writer bias was not there, and a
+ * program had a large number of threads, STW-induced pauses could be
+ * significantly longer than they have to be. Also, we emit periodic
+ * sync points from the helper thread, which requires taking the
+ * exclusive lock, and we need those to arrive with a reasonably
+ * consistent frequency so that readers don't have to queue up too many
+ * events between sync points.
+ */
+static volatile gint32 buffer_lock_state;
+static volatile gint32 buffer_lock_exclusive_intent;
+
+// Can be used recursively.
+static void
+buffer_lock (void)
+{
+ /*
+ * If the thread holding the exclusive lock tries to modify the
+ * reader count, just make it a no-op. This way, we also avoid
+ * invoking the GC safe point macros below, which could break if
+ * done from a thread that is currently the initiator of STW.
+ *
+ * In other words, we rely on the fact that the GC thread takes
+ * the exclusive lock in the gc_event () callback when the world
+ * is about to stop.
+ */
+ if (InterlockedRead (&buffer_lock_state) != get_thread ()->small_id << 16) {
+ MONO_ENTER_GC_SAFE;
+
+ gint32 old, new_;
+
+ do {
+ restart:
+ // Hold off if a thread wants to take the exclusive lock.
+ while (InterlockedRead (&buffer_lock_exclusive_intent))
+ mono_thread_info_yield ();
+
+ old = InterlockedRead (&buffer_lock_state);
+
+ // Is a thread holding the exclusive lock?
+ if (old >> 16) {
+ mono_thread_info_yield ();
+ goto restart;
+ }
+
+ new_ = old + 1;
+ } while (InterlockedCompareExchange (&buffer_lock_state, new_, old) != old);
+
+ MONO_EXIT_GC_SAFE;
+ }
+
+ mono_memory_barrier ();
+}
+
+static void
+buffer_unlock (void)
+{
+ mono_memory_barrier ();
+
+ gint32 state = InterlockedRead (&buffer_lock_state);
+
+ // See the comment in buffer_lock ().
+ if (state == PROF_TLS_GET ()->small_id << 16)
+ return;
+
+ g_assert (state && "Why are we decrementing a zero reader count?");
+ g_assert (!(state >> 16) && "Why is the exclusive lock held?");
+
+ InterlockedDecrement (&buffer_lock_state);
+}
+
+// Cannot be used recursively.
+static void
+buffer_lock_excl (void)
+{
+ gint32 new_ = get_thread ()->small_id << 16;
+
+ g_assert (InterlockedRead (&buffer_lock_state) != new_ && "Why are we taking the exclusive lock twice?");
+
+ InterlockedIncrement (&buffer_lock_exclusive_intent);
+
+ MONO_ENTER_GC_SAFE;
+
+ while (InterlockedCompareExchange (&buffer_lock_state, new_, 0))
+ mono_thread_info_yield ();
+
+ MONO_EXIT_GC_SAFE;
+
+ mono_memory_barrier ();
+}
+
+static void
+buffer_unlock_excl (void)
+{
+ mono_memory_barrier ();
+
+ gint32 state = InterlockedRead (&buffer_lock_state);
+ gint32 excl = state >> 16;
+
+ g_assert (excl && "Why is the exclusive lock not held?");
+ g_assert (excl == PROF_TLS_GET ()->small_id && "Why does another thread hold the exclusive lock?");
+ g_assert (!(state & 0xFFFF) && "Why are there readers when the exclusive lock is held?");
+
+ InterlockedWrite (&buffer_lock_state, 0);
+ InterlockedDecrement (&buffer_lock_exclusive_intent);
+}
+
static void
encode_uleb128 (uint64_t value, uint8_t *buf, uint8_t **endbuf)
{
static void
sync_point_flush (void)
{
- g_assert (InterlockedReadPointer (&buffer_rwlock_exclusive) == (gpointer) thread_id () && "Why don't we hold the exclusive lock?");
+ g_assert (InterlockedRead (&buffer_lock_state) == PROF_TLS_GET ()->small_id << 16 && "Why don't we hold the exclusive lock?");
MONO_LLS_FOREACH_SAFE (&profiler_thread_list, MonoProfilerThread, thread) {
g_assert (thread->attached && "Why is a thread in the LLS not attached?");
static void
sync_point_mark (MonoProfilerSyncPointType type)
{
- g_assert (InterlockedReadPointer (&buffer_rwlock_exclusive) == (gpointer) thread_id () && "Why don't we hold the exclusive lock?");
+ g_assert (InterlockedRead (&buffer_lock_state) == PROF_TLS_GET ()->small_id << 16 && "Why don't we hold the exclusive lock?");
ENTER_LOG (&sync_points_ctr, logbuffer,
EVENT_SIZE /* event */ +
*/
mono_thread_hazardous_try_free_all ();
- g_assert (!InterlockedRead (&buffer_rwlock_count) && "Why is the reader count still non-zero?");
- g_assert (!InterlockedReadPointer (&buffer_rwlock_exclusive) && "Why does someone still hold the exclusive lock?");
+ gint32 state = InterlockedRead (&buffer_lock_state);
+
+ g_assert (!(state & 0xFFFF) && "Why is the reader count still non-zero?");
+ g_assert (!(state >> 16) && "Why is the exclusive lock still held?");
#if defined (HAVE_SYS_ZLIB)
if (prof->gzfile)
projects / mono.git / commitdiff