monitor.c

/*
 * monitor.c:  Monitor locking functions
 *
 * Author:
 *	Dick Porter (dick@ximian.com)
 *
 * (C) 2003 Ximian, Inc.
 */

#include <config.h>
#include <glib.h>

#include <mono/metadata/monitor.h>
#include <mono/metadata/threads-types.h>
#include <mono/metadata/exception.h>
#include <mono/metadata/threads.h>
#include <mono/io-layer/io-layer.h>
#include <mono/metadata/object-internals.h>
#include <mono/metadata/gc-internal.h>

/*#define LOCK_DEBUG(a) do { a; } while (0)*/
#define LOCK_DEBUG(a)

/*
 * The monitor implementation here is based on
 * http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf and
 * http://www.research.ibm.com/people/d/dfb/papers/Bacon98Thin.ps
 *
 * The Dice paper describes a technique for saving lock record space
 * by returning records to a free list when they become unused.  That
 * sounds like unnecessary complexity to me, though if it becomes
 * clear that unused lock records are taking up lots of space or we
 * need to shave more time off by avoiding a malloc then we can always
 * implement the free list idea later.  The timeout parameter to
 * try_enter voids some of the assumptions about the reference count
 * field in Dice's implementation too.  In his version, the thread
 * attempting to lock a contended object will block until it succeeds,
 * so the reference count will never be decremented while an object is
 * locked.
 *
 * Bacon's thin locks have a fast path that doesn't need a lock record
 * for the common case of locking an unlocked or shallow-nested
 * object, but the technique relies on encoding the thread ID in 15
 * bits (to avoid too much per-object space overhead.)  Unfortunately
 * I don't think it's possible to reliably encode a pthread_t into 15
 * bits. (The JVM implementation used seems to have a 15-bit
 * per-thread identifier available.)
 *
 * This implementation then combines Dice's basic lock model with
 * Bacon's simplification of keeping a lock record for the lifetime of
 * an object.
 */

struct _MonoThreadsSync
{
	guint32 owner;			/* thread ID */
	guint32 nest;
	volatile guint32 entry_count;
	HANDLE entry_sem;
	GSList *wait_list;
	void *data;
};

typedef struct _MonitorArray MonitorArray;

struct _MonitorArray {
	MonitorArray *next;
	int num_monitors;
	MonoThreadsSync monitors [MONO_ZERO_LEN_ARRAY];
};

static CRITICAL_SECTION monitor_mutex;
static MonoThreadsSync *monitor_freelist;
static MonitorArray *monitor_allocated;
static int array_size = 16;

void
mono_monitor_init (void)
{
	InitializeCriticalSection (&monitor_mutex);
}

/* LOCKING: this is called with monitor_mutex held */
static void 
mon_finalize (MonoThreadsSync *mon)
{
	LOCK_DEBUG (g_message (G_GNUC_PRETTY_FUNCTION ": Finalizing sync %p", mon));

	if (mon->entry_sem != NULL) {
		CloseHandle (mon->entry_sem);
		mon->entry_sem = NULL;
	}
	/* If this isn't empty then something is seriously broken - it
	 * means a thread is still waiting on the object that owned
	 * this lock, but the object has been finalized.
	 */
	g_assert (mon->wait_list == NULL);

	mon->entry_count = 0;
	/* owner and nest are set in mon_new, no need to zero them out */

	mon->data = monitor_freelist;
	monitor_freelist = mon;
}

/* LOCKING: this is called with monitor_mutex held */
static MonoThreadsSync *
mon_new (guint32 id)
{
	MonoThreadsSync *new;

	if (!monitor_freelist) {
		MonitorArray *marray;
		int i;
		/* see if any sync block has been collected */
		new = NULL;
		for (marray = monitor_allocated; marray; marray = marray->next) {
			for (i = 0; i < marray->num_monitors; ++i) {
				if (marray->monitors [i].data == NULL) {
					new = &marray->monitors [i];
					new->data = monitor_freelist;
					monitor_freelist = new;
				}
			}
			/* small perf tweak to avoid scanning all the blocks */
			if (new)
				break;
		}
		/* need to allocate a new array of monitors */
		if (!monitor_freelist) {
			MonitorArray *last;
			LOCK_DEBUG (g_message (G_GNUC_PRETTY_FUNCTION ": allocating more monitors: %d", array_size));
			marray = g_malloc0 (sizeof (MonoArray) + array_size * sizeof (MonoThreadsSync));
			marray->num_monitors = array_size;
			array_size *= 2;
			/* link into the freelist */
			for (i = 0; i < marray->num_monitors - 1; ++i) {
				marray->monitors [i].data = &marray->monitors [i + 1];
			}
			marray->monitors [i].data = NULL; /* the last one */
			monitor_freelist = &marray->monitors [0];
			/* we happend the marray instead of prepending so that
			 * the collecting loop above will need to scan smaller arrays first
			 */
			if (!monitor_allocated) {
				monitor_allocated = marray;
			} else {
				last = monitor_allocated;
				while (last->next)
					last = last->next;
				last->next = marray;
			}
		}
	}

	new = monitor_freelist;
	monitor_freelist = new->data;

	new->owner = id;
	new->nest = 1;
	
	return new;
}

/* If allow_interruption==TRUE, the method will be interrumped if abort or suspend
 * is requested. In this case it returns -1.
 */ 
static gint32 
mono_monitor_try_enter_internal (MonoObject *obj, guint32 ms, gboolean allow_interruption)
{
	MonoThreadsSync *mon;
	guint32 id = GetCurrentThreadId ();
	HANDLE sem;
	guint32 then = 0, now, delta;
	guint32 waitms;
	guint32 ret;
	
	LOCK_DEBUG (g_message(G_GNUC_PRETTY_FUNCTION
		  ": (%d) Trying to lock object %p (%d ms)", id, obj, ms));

retry:
	mon = obj->synchronisation;

	/* If the object has never been locked... */
	if (mon == NULL) {
		EnterCriticalSection (&monitor_mutex);
		mon = mon_new (id);
		if (InterlockedCompareExchangePointer ((gpointer*)&obj->synchronisation, mon, NULL) == NULL) {
			mono_gc_weak_link_add (&mon->data, obj);
			LeaveCriticalSection (&monitor_mutex);
			/* Successfully locked */
			return 1;
		} else {
			mon_finalize (mon);
			LeaveCriticalSection (&monitor_mutex);
			goto retry;
		}
	}

	/* If the object is currently locked by this thread... */
	if (mon->owner == id) {
		mon->nest++;
		return 1;
	}

	/* If the object has previously been locked but isn't now... */

	/* This case differs from Dice's case 3 because we don't
	 * deflate locks or cache unused lock records
	 */
	if (mon->owner == 0) {
		/* Try to install our ID in the owner field, nest
		 * should have been left at 1 by the previous unlock
		 * operation
		 */
		if (InterlockedCompareExchange (&mon->owner, id, 0) == 0) {
			/* Success */
			g_assert (mon->nest == 1);
			return 1;
		} else {
			/* Trumped again! */
			goto retry;
		}
	}

	/* The object must be locked by someone else... */

	/* If ms is 0 we don't block, but just fail straight away */
	if (ms == 0) {
		LOCK_DEBUG (g_message (G_GNUC_PRETTY_FUNCTION ": (%d) timed out, returning FALSE", id));
		return 0;
	}

	/* The slow path begins here.  We need to make sure theres a
	 * semaphore handle (creating it if necessary), and block on
	 * it
	 */
	if (mon->entry_sem == NULL) {
		/* Create the semaphore */
		sem = CreateSemaphore (NULL, 0, 0x7fffffff, NULL);
		if (InterlockedCompareExchangePointer ((gpointer*)&mon->entry_sem, sem, NULL) != NULL) {
			/* Someone else just put a handle here */
			CloseHandle (sem);
		}
	}

	/* If we need to time out, record a timestamp and adjust ms,
	 * because WaitForSingleObject doesn't tell us how long it
	 * waited for.
	 *
	 * Don't block forever here, because theres a chance the owner
	 * thread released the lock while we were creating the
	 * semaphore: we would not get the wakeup.  Using the event
	 * handle technique from pulse/wait would involve locking the
	 * lock struct and therefore slowing down the fast path.
	 */
	if (ms != INFINITE) {
		then = GetTickCount ();
		if (ms < 100) {
			waitms = ms;
		} else {
			waitms = 100;
		}
	} else {
		waitms = 100;
	}
	
	InterlockedIncrement (&mon->entry_count);
	ret = WaitForSingleObjectEx (mon->entry_sem, waitms, allow_interruption);
	InterlockedDecrement (&mon->entry_count);

	if (ms != INFINITE) {
		now = GetTickCount ();
		
		if (now < then) {
			/* The counter must have wrapped around */
			LOCK_DEBUG (g_message (G_GNUC_PRETTY_FUNCTION
				   ": wrapped around! now=0x%x then=0x%x", now, then));
			
			now += (0xffffffff - then);
			then = 0;

			LOCK_DEBUG (g_message (G_GNUC_PRETTY_FUNCTION ": wrap rejig: now=0x%x then=0x%x delta=0x%x", now, then, now-then));
		}
		
		delta = now - then;
		if (delta >= ms) {
			ms = 0;
		} else {
			ms -= delta;
		}

		if ((ret == WAIT_TIMEOUT || (ret == WAIT_IO_COMPLETION && !allow_interruption)) && ms > 0) {
			/* More time left */
			goto retry;
		}
	} else {
		if (ret == WAIT_TIMEOUT || (ret == WAIT_IO_COMPLETION && !allow_interruption)) {
			/* Infinite wait, so just try again */
			goto retry;
		}
	}
	
	if (ret == WAIT_OBJECT_0) {
		/* retry from the top */
		goto retry;
	}
	
	/* We must have timed out */
	LOCK_DEBUG (g_message (G_GNUC_PRETTY_FUNCTION ": (%d) timed out waiting, returning FALSE", id));

	if (ret == WAIT_IO_COMPLETION)
		return -1;
	else 
		return 0;
}

gboolean 
mono_monitor_enter (MonoObject *obj)
{
	return mono_monitor_try_enter_internal (obj, INFINITE, FALSE) == 1;
}

gboolean 
mono_monitor_try_enter (MonoObject *obj, guint32 ms)
{
	return mono_monitor_try_enter_internal (obj, ms, FALSE) == 1;
}

void 
mono_monitor_exit (MonoObject *obj)
{
	MonoThreadsSync *mon;
	guint32 nest;
	
	LOCK_DEBUG (g_message (G_GNUC_PRETTY_FUNCTION ": (%d) Unlocking %p", GetCurrentThreadId (), obj));

	mon = obj->synchronisation;

	if (mon == NULL) {
		mono_raise_exception (mono_get_exception_synchronization_lock ("Not locked"));
		return;
	}
	if (mon->owner != GetCurrentThreadId ()) {
		return;
	}
	
	nest = mon->nest - 1;
	if (nest == 0) {
		LOCK_DEBUG (g_message (G_GNUC_PRETTY_FUNCTION
			  ": (%d) Object %p is now unlocked", GetCurrentThreadId (), obj));
	
		/* object is now unlocked, leave nest==1 so we don't
		 * need to set it when the lock is reacquired
		 */
		mon->owner = 0;

		/* Do the wakeup stuff.  It's possible that the last
		 * blocking thread gave up waiting just before we
		 * release the semaphore resulting in a futile wakeup
		 * next time there's contention for this object, but
		 * it means we don't have to waste time locking the
		 * struct.
		 */
		if (mon->entry_count > 0) {
			ReleaseSemaphore (mon->entry_sem, 1, NULL);
		}
	} else {
		LOCK_DEBUG (g_message (G_GNUC_PRETTY_FUNCTION
			  ": (%d) Object %p is now locked %d times", GetCurrentThreadId (), obj, nest));
		mon->nest = nest;
	}
}

gboolean 
ves_icall_System_Threading_Monitor_Monitor_try_enter (MonoObject *obj, guint32 ms)
{
	gint32 res;

	do {
		res = mono_monitor_try_enter_internal (obj, ms, TRUE);
		if (res == -1)
			mono_thread_interruption_checkpoint ();
	} while (res == -1);
	
	return res == 1;
}

void 
ves_icall_System_Threading_Monitor_Monitor_exit (MonoObject *obj)
{
	mono_monitor_exit (obj);
}

gboolean 
ves_icall_System_Threading_Monitor_Monitor_test_owner (MonoObject *obj)
{
	MonoThreadsSync *mon;
	
	LOCK_DEBUG (g_message (G_GNUC_PRETTY_FUNCTION
		  ": Testing if %p is owned by thread %d", obj, GetCurrentThreadId()));

	mon = obj->synchronisation;
	if (mon == NULL) {
		return FALSE;
	}
	
	if(mon->owner==GetCurrentThreadId ()) {
		return(TRUE);
	}
	
	return(FALSE);
}

gboolean 
ves_icall_System_Threading_Monitor_Monitor_test_synchronised (MonoObject *obj)
{
	MonoThreadsSync *mon;

	LOCK_DEBUG (g_message(G_GNUC_PRETTY_FUNCTION
		  ": (%d) Testing if %p is owned by any thread", GetCurrentThreadId (), obj));
	
	mon = obj->synchronisation;
	if (mon == NULL) {
		return FALSE;
	}
	
	if (mon->owner != 0) {
		return TRUE;
	}
	
	return FALSE;
}

/* All wait list manipulation in the pulse, pulseall and wait
 * functions happens while the monitor lock is held, so we don't need
 * any extra struct locking
 */

void
ves_icall_System_Threading_Monitor_Monitor_pulse (MonoObject *obj)
{
	MonoThreadsSync *mon;
	
	LOCK_DEBUG (g_message (G_GNUC_PRETTY_FUNCTION ": (%d) Pulsing %p", 
		GetCurrentThreadId (), obj));
	
	mon = obj->synchronisation;
	if (mon == NULL) {
		mono_raise_exception (mono_get_exception_synchronization_lock ("Not locked"));
		return;
	}
	if (mon->owner != GetCurrentThreadId ()) {
		mono_raise_exception (mono_get_exception_synchronization_lock ("Not locked by this thread"));
		return;
	}

	LOCK_DEBUG (g_message (G_GNUC_PRETTY_FUNCTION ": (%d) %d threads waiting",
		  GetCurrentThreadId (), g_slist_length (mon->wait_list)));
	
	if (mon->wait_list != NULL) {
		LOCK_DEBUG (g_message (G_GNUC_PRETTY_FUNCTION
			  ": (%d) signalling and dequeuing handle %p",
			  GetCurrentThreadId (), mon->wait_list->data));
	
		SetEvent (mon->wait_list->data);
		mon->wait_list = g_slist_remove (mon->wait_list, mon->wait_list->data);
	}
}

void
ves_icall_System_Threading_Monitor_Monitor_pulse_all (MonoObject *obj)
{
	MonoThreadsSync *mon;
	
	LOCK_DEBUG (g_message(G_GNUC_PRETTY_FUNCTION ": (%d) Pulsing all %p",
		  GetCurrentThreadId (), obj));

	mon = obj->synchronisation;
	if (mon == NULL) {
		mono_raise_exception (mono_get_exception_synchronization_lock ("Not locked"));
		return;
	}
	if (mon->owner != GetCurrentThreadId ()) {
		mono_raise_exception (mono_get_exception_synchronization_lock ("Not locked by this thread"));
		return;
	}

	LOCK_DEBUG (g_message (G_GNUC_PRETTY_FUNCTION ": (%d) %d threads waiting",
		  GetCurrentThreadId (), g_slist_length (mon->wait_list)));

	while (mon->wait_list != NULL) {
		LOCK_DEBUG (g_message (G_GNUC_PRETTY_FUNCTION
			  ": (%d) signalling and dequeuing handle %p",
			  GetCurrentThreadId (), mon->wait_list->data));
	
		SetEvent (mon->wait_list->data);
		mon->wait_list = g_slist_remove (mon->wait_list, mon->wait_list->data);
	}
}

gboolean
ves_icall_System_Threading_Monitor_Monitor_wait (MonoObject *obj, guint32 ms)
{
	MonoThreadsSync *mon;
	HANDLE event;
	guint32 nest;
	guint32 ret;
	gboolean success = FALSE;
	gint32 regain;
	MonoThread *thread = mono_thread_current ();

	LOCK_DEBUG (g_message (G_GNUC_PRETTY_FUNCTION
		  ": (%d) Trying to wait for %p with timeout %dms",
		  GetCurrentThreadId (), obj, ms));
	
	mon = obj->synchronisation;
	if (mon == NULL) {
		mono_raise_exception (mono_get_exception_synchronization_lock ("Not locked"));
		return FALSE;
	}
	if (mon->owner != GetCurrentThreadId ()) {
		mono_raise_exception (mono_get_exception_synchronization_lock ("Not locked by this thread"));
		return FALSE;
	}
	
	event = CreateEvent (NULL, FALSE, FALSE, NULL);
	if (event == NULL) {
		mono_raise_exception (mono_get_exception_synchronization_lock ("Failed to set up wait event"));
		return FALSE;
	}
	
	LOCK_DEBUG (g_message (G_GNUC_PRETTY_FUNCTION ": (%d) queuing handle %p",
		  GetCurrentThreadId (), event));

	mono_monitor_enter (thread->synch_lock);
	thread->state |= ThreadState_WaitSleepJoin;
	mono_monitor_exit (thread->synch_lock);

	mon->wait_list = g_slist_append (mon->wait_list, event);
	
	/* Save the nest count, and release the lock */
	nest = mon->nest;
	mon->nest = 1;
	mono_monitor_exit (obj);

	LOCK_DEBUG (g_message (G_GNUC_PRETTY_FUNCTION ": (%d) Unlocked %p lock %p",
		  GetCurrentThreadId (), obj, mon));

	/* There's no race between unlocking mon and waiting for the
	 * event, because auto reset events are sticky, and this event
	 * is private to this thread.  Therefore even if the event was
	 * signalled before we wait, we still succeed.
	 */
	ret = WaitForSingleObjectEx (event, ms, TRUE);

	/* Reset the thread state fairly early, so we don't have to worry
	 * about the monitor error checking
	 */
	mono_monitor_enter (thread->synch_lock);
	thread->state &= ~ThreadState_WaitSleepJoin;
	mono_monitor_exit (thread->synch_lock);
	
	if (mono_thread_interruption_requested ()) {
		CloseHandle (event);
		return FALSE;
	}

	/* Regain the lock with the previous nest count */
	do {
		regain = mono_monitor_try_enter_internal (obj, INFINITE, TRUE);
		if (regain == -1) 
			mono_thread_interruption_checkpoint ();
	} while (regain == -1);

	if (regain == 0) {
		/* Something went wrong, so throw a
		 * SynchronizationLockException
		 */
		CloseHandle (event);
		mono_raise_exception (mono_get_exception_synchronization_lock ("Failed to regain lock"));
		return FALSE;
	}

	mon->nest = nest;

	LOCK_DEBUG (g_message (G_GNUC_PRETTY_FUNCTION ": (%d) Regained %p lock %p",
		  GetCurrentThreadId (), obj, mon));

	if (ret == WAIT_TIMEOUT) {
		/* Poll the event again, just in case it was signalled
		 * while we were trying to regain the monitor lock
		 */
		ret = WaitForSingleObjectEx (event, 0, FALSE);
	}

	/* Pulse will have popped our event from the queue if it signalled
	 * us, so we only do it here if the wait timed out.
	 *
	 * This avoids a race condition where the thread holding the
	 * lock can Pulse several times before the WaitForSingleObject
	 * returns.  If we popped the queue here then this event might
	 * be signalled more than once, thereby starving another
	 * thread.
	 */
	
	if (ret == WAIT_OBJECT_0) {
		LOCK_DEBUG (g_message (G_GNUC_PRETTY_FUNCTION ": (%d) Success",
			  GetCurrentThreadId ()));
		success = TRUE;
	} else {
		LOCK_DEBUG (g_message (G_GNUC_PRETTY_FUNCTION ": (%d) Wait failed, dequeuing handle %p",
			  GetCurrentThreadId (), event));
		/* No pulse, so we have to remove ourself from the
		 * wait queue
		 */
		mon->wait_list = g_slist_remove (mon->wait_list, event);
	}
	CloseHandle (event);
	
	return success;
}