Index: libc/generic/futex.c
===================================================================
--- libc/generic/futex.c	(revision 77bd0042650371be594f68132406e2c863e3dc6f)
+++ libc/generic/futex.c	(revision 17242c6e5b745ef9ae07c90206932c94ce0ec40c)
@@ -31,5 +31,48 @@
 #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)
 {
@@ -39,13 +82,79 @@
 int futex_down(atomic_t *futex)
 {
-	long val;
-	
-	val = atomic_predec(futex);
-	if (val < 0)
-		return __SYSCALL1(SYS_FUTEX_SLEEP, (sysarg_t) &futex->count);
-		
-	return 0;
+	return futex_down_timeout(futex, SYNCH_NO_TIMEOUT, SYNCH_BLOCKING);
 }
 
+int futex_trydown(atomic_t *futex)
+{
+	return futex_down_timeout(futex, SYNCH_NO_TIMEOUT, SYNCH_NON_BLOCKING);
+}
+
+/** Try to down the futex.
+ *
+ * @param futex Futex.
+ * @param usec Microseconds to wait. Zero value means sleep without timeout.
+ * @param trydown If usec is zero and trydown is non-zero, only conditional
+ *
+ * @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 trydown)
+{
+	int rc;
+	
+	while (atomic_predec(futex) < 0) {
+		rc = __SYSCALL3(SYS_FUTEX_SLEEP, (sysarg_t) &futex->count, (sysarg_t) usec, (sysarg_t) trydown);
+		
+		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 or futex. Otherwise zero.
+ */
 int futex_up(atomic_t *futex)
 {
