From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Date: Thu, 9 Jun 2011 14:39:56 +0000 (-0400)
Subject: call_rcu: handle retry without wait correctly
X-Git-Tag: v0.6.0~25
X-Git-Url: https://git.lttng.org./?a=commitdiff_plain;h=bc94ca9bada25f7403e3e859caa241146ae8e338;p=userspace-rcu.git

call_rcu: handle retry without wait correctly

The wait scheme has an implementation problem: if the list is not empty
when the !RT scheme checks for it, it will restart the loop and
decrement the futex (again) without calling call_rcu_wait() (which would
wait until it is set back to 0). So in this case, we can end up
decrementing "futex" to values well below -1.

Fix this by moving the decrement before the loop, and duplicate it after
return from call_rcu_wait() + poll() delay. Also move the "set futex to
0 upon stopping" outside of the loop: this is the only way the loop can
be stopped anyway.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
---

diff --git a/urcu-call-rcu-impl.h b/urcu-call-rcu-impl.h
index 82fec80..6fcf8c8 100644
--- a/urcu-call-rcu-impl.h
+++ b/urcu-call-rcu-impl.h
@@ -211,12 +211,12 @@ static void *call_rcu_thread(void *arg)
 	}
 
 	thread_call_rcu_data = crdp;
+	if (!rt) {
+		uatomic_dec(&crdp->futex);
+		/* Decrement futex before reading call_rcu list */
+		cmm_smp_mb();
+	}
 	for (;;) {
-		if (!rt) {
-			uatomic_dec(&crdp->futex);
-			/* Decrement futex before reading call_rcu list */
-			cmm_smp_mb();
-		}
 		if (&crdp->cbs.head != _CMM_LOAD_SHARED(crdp->cbs.tail)) {
 			while ((cbs = _CMM_LOAD_SHARED(crdp->cbs.head)) == NULL)
 				poll(NULL, 0, 1);
@@ -240,21 +240,30 @@ static void *call_rcu_thread(void *arg)
 			} while (cbs != NULL);
 			uatomic_sub(&crdp->qlen, cbcount);
 		}
-		if (uatomic_read(&crdp->flags) & URCU_CALL_RCU_STOP) {
-			if (!rt) {
+		if (uatomic_read(&crdp->flags) & URCU_CALL_RCU_STOP)
+			break;
+		if (!rt) {
+			if (&crdp->cbs.head
+			    == _CMM_LOAD_SHARED(crdp->cbs.tail)) {
+				call_rcu_wait(crdp);
+				poll(NULL, 0, 10);
+				uatomic_dec(&crdp->futex);
 				/*
-				 * Read call_rcu list before write futex.
+				 * Decrement futex before reading
+				 * call_rcu list.
 				 */
 				cmm_smp_mb();
-				uatomic_set(&crdp->futex, 0);
 			}
-			break;
-		}
-		if (!rt) {
-			if (&crdp->cbs.head == _CMM_LOAD_SHARED(crdp->cbs.tail))
-				call_rcu_wait(crdp);
+		} else {
+			poll(NULL, 0, 10);
 		}
-		poll(NULL, 0, 10);
+	}
+	if (!rt) {
+		/*
+		 * Read call_rcu list before write futex.
+		 */
+		cmm_smp_mb();
+		uatomic_set(&crdp->futex, 0);
 	}
 	uatomic_or(&crdp->flags, URCU_CALL_RCU_STOPPED);
 	return NULL;