Index: uspace/libc/generic/futex.c
===================================================================
--- uspace/libc/generic/futex.c	(revision 0166e99928c90c6d2e990c69a7aaf84a6f93c6c0)
+++ uspace/libc/generic/futex.c	(revision 0166e99928c90c6d2e990c69a7aaf84a6f93c6c0)
@@ -0,0 +1,180 @@
+/*
+ * Copyright (C) 2006 Jakub Jermar
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *
+ * - Redistributions of source code must retain the above copyright
+ *   notice, this list of conditions and the following disclaimer.
+ * - Redistributions in binary form must reproduce the above copyright
+ *   notice, this list of conditions and the following disclaimer in the
+ *   documentation and/or other materials provided with the distribution.
+ * - The name of the author may not be used to endorse or promote products
+ *   derived from this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
+ * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+ * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
+ * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+ * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
+ * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+ /** @addtogroup libc
+ * @{
+ */
+/** @file
+ */ 
+
+#include <futex.h>
+#include <atomic.h>
+#include <libc.h>
+#include <stdio.h>
+#include <types.h>
+#include <kernel/synch/synch.h>
+
+/*
+ * Note about race conditions.
+ * Because of non-atomic nature of operations performed sequentially on the futex
+ * counter and the futex wait queue, there is a race condition:
+ *
+ * (wq->missed_wakeups == 1) && (futex->count = 1)
+ *
+ * Scenario 1 (wait queue timeout vs. futex_up()):
+ * 1. assume wq->missed_wakeups == 0 && futex->count == -1
+ *    (ie. thread A sleeping, thread B in the critical section)
+ * 2. A receives timeout and gets removed from the wait queue
+ * 3. B wants to leave the critical section and calls futex_up()
+ * 4. B thus changes futex->count from -1 to 0
+ * 5. B has to call SYS_FUTEX_WAKEUP syscall to wake up the sleeping thread
+ * 6. B finds the wait queue empty and changes wq->missed_wakeups from 0 to 1
+ * 7. A fixes futex->count (i.e. the number of waiting threads) by changing it from 0 to 1
+ *
+ * Scenario 2 (conditional down operation vs. futex_up)
+ * 1. assume wq->missed_wakeups == 0 && futex->count == 0
+ *    (i.e. thread A is in the critical section)
+ * 2. thread B performs futex_trydown() operation and changes futex->count from 0 to -1
+ *    B is now obliged to call SYS_FUTEX_SLEEP syscall
+ * 3. A wants to leave the critical section and does futex_up()
+ * 4. A thus changes futex->count from -1 to 0 and must call SYS_FUTEX_WAKEUP syscall
+ * 5. B finds the wait queue empty and immediatelly aborts the conditional sleep
+ * 6. No thread is queueing in the wait queue so wq->missed_wakeups changes from 0 to 1
+ * 6. B fixes futex->count (i.e. the number of waiting threads) by changing it from 0 to 1
+ *
+ * Both scenarios allow two threads to be in the critical section simultaneously.
+ * One without kernel intervention and the other through wq->missed_wakeups being 1.
+ *
+ * To mitigate this problem, futex_down_timeout() detects that the syscall didn't sleep
+ * in the wait queue, fixes the futex counter and RETRIES the whole operation again.
+ *
+ */
+
+/** Initialize futex counter.
+ *
+ * @param futex Futex.
+ * @param val Initialization value.
+ */
+void futex_initialize(atomic_t *futex, int val)
+{
+	atomic_set(futex, val);
+}
+
+int futex_down(atomic_t *futex)
+{
+	return futex_down_timeout(futex, SYNCH_NO_TIMEOUT, SYNCH_FLAGS_NONE);
+}
+
+int futex_trydown(atomic_t *futex)
+{
+	return futex_down_timeout(futex, SYNCH_NO_TIMEOUT, SYNCH_FLAGS_NON_BLOCKING);
+}
+
+/** Try to down the futex.
+ *
+ * @param futex Futex.
+ * @param usec Microseconds to wait. Zero value means sleep without timeout.
+ * @param flags Select mode of operation. See comment for waitq_sleep_timeout(). 
+ *
+ * @return ENOENT if there is no such virtual address. One of ESYNCH_OK_ATOMIC
+ *	   and ESYNCH_OK_BLOCKED on success or ESYNCH_TIMEOUT if the lock was
+ *	   not acquired because of a timeout or ESYNCH_WOULD_BLOCK if the
+ *	   operation could not be carried out atomically (if requested so).
+ */
+int futex_down_timeout(atomic_t *futex, uint32_t usec, int flags)
+{
+	int rc;
+	
+	while (atomic_predec(futex) < 0) {
+		rc = __SYSCALL3(SYS_FUTEX_SLEEP, (sysarg_t) &futex->count, (sysarg_t) usec, (sysarg_t) flags);
+		
+		switch (rc) {
+		case ESYNCH_OK_ATOMIC:
+			/*
+			 * Because of a race condition between timeout and futex_up()
+			 * and between conditional futex_down_timeout() and futex_up(),
+			 * we have to give up and try again in this special case.
+			 */
+			atomic_inc(futex);
+			break;
+
+		case ESYNCH_TIMEOUT:
+			atomic_inc(futex);
+			return ESYNCH_TIMEOUT;
+			break;
+
+		case ESYNCH_WOULD_BLOCK:
+			/*
+			 * The conditional down operation should be implemented this way.
+			 * The userspace-only variant tends to accumulate missed wakeups
+			 * in the kernel futex wait queue.
+			 */
+			atomic_inc(futex);
+			return ESYNCH_WOULD_BLOCK;
+			break;
+
+		case ESYNCH_OK_BLOCKED:
+			/*
+			 * Enter the critical section.
+			 * The futex counter has already been incremented for us.
+			 */
+			return ESYNCH_OK_BLOCKED;
+			break;
+		default:
+			return rc;
+		}
+	}
+
+	/*
+	 * Enter the critical section.
+	 */
+	return ESYNCH_OK_ATOMIC;
+}
+
+/** Up the futex.
+ *
+ * @param futex Futex.
+ *
+ * @return ENOENT if there is no such virtual address. Otherwise zero.
+ */
+int futex_up(atomic_t *futex)
+{
+	long val;
+	
+	val = atomic_postinc(futex);
+	if (val < 0)
+		return __SYSCALL1(SYS_FUTEX_WAKEUP, (sysarg_t) &futex->count);
+		
+	return 0;
+}
+
+
+ /** @}
+ */
+ 
+ 
