Contributions from XFS for FreeBSD project:

- Implement cv_wait_unlock() method which has semantics compatible
  with the sv_wait() method in IRIX.  For cv_wait_unlock(), the lock
  must be held before entering the function, but is not held when the
  function is exited.

- Implement the existing cv_wait() function in terms of cv_wait_unlock().

Submitted by:	kan
Feedback from:	jhb, trhodes, Christoph Hellwig <hch at infradead dot org>

2 files changed, 28 insertions, 8 deletions

diff --git a/sys/kern/kern_condvar.c b/sys/kern/kern_condvar.c
index 8ccea3a..ede6b61 100644
--- a/sys/kern/kern_condvar.c
+++ b/sys/kern/kern_condvar.c
@@ -95,9 +95,34 @@ cv_destroy(struct cv *cvp)
 void
 cv_wait(struct cv *cvp, struct mtx *mp)
 {
-	struct thread *td;
 	WITNESS_SAVE_DECL(mp);
 
+	WITNESS_SAVE(&mp->mtx_object, mp);
+
+	if (cold || panicstr) {
+		/*
+		 * During autoconfiguration, just give interrupts
+		 * a chance, then just return.  Don't run any other
+		 * thread or panic below, in case this is the idle
+		 * process and already asleep.
+		 */
+		return;
+	}
+
+	cv_wait_unlock(cvp, mp);
+	mtx_lock(mp);
+	WITNESS_RESTORE(&mp->mtx_object, mp);
+}
+
+/*
+ * Wait on a condition variable.  This function differs from cv_wait by
+ * not aquiring the mutex after condition variable was signaled.
+ */
+void
+cv_wait_unlock(struct cv *cvp, struct mtx *mp)
+{
+	struct thread *td;
+
 	td = curthread;
 #ifdef KTRACE
 	if (KTRPOINT(td, KTR_CSW))
@@ -106,7 +131,6 @@ cv_wait(struct cv *cvp, struct mtx *mp)
 	CV_ASSERT(cvp, mp, td);
 	WITNESS_WARN(WARN_GIANTOK | WARN_SLEEPOK, &mp->mtx_object,
 	    "Waiting on \"%s\"", cvp->cv_description);
-	WITNESS_SAVE(&mp->mtx_object, mp);
 
 	if (cold || panicstr) {
 		/*
@@ -115,6 +139,7 @@ cv_wait(struct cv *cvp, struct mtx *mp)
 		 * thread or panic below, in case this is the idle
 		 * process and already asleep.
 		 */
+		mtx_unlock(mp);
 		return;
 	}
 
@@ -127,13 +152,7 @@ cv_wait(struct cv *cvp, struct mtx *mp)
 	sleepq_add(cvp, mp, cvp->cv_description, SLEEPQ_CONDVAR);
 	sleepq_wait(cvp);
 
-#ifdef KTRACE
-	if (KTRPOINT(td, KTR_CSW))
-		ktrcsw(0, 0);
-#endif
 	PICKUP_GIANT();
-	mtx_lock(mp);
-	WITNESS_RESTORE(&mp->mtx_object, mp);
 }
 
 /*
diff --git a/sys/sys/condvar.h b/sys/sys/condvar.h
index a8c9db7..97f699a 100644
--- a/sys/sys/condvar.h
+++ b/sys/sys/condvar.h
@@ -53,6 +53,7 @@ void	cv_init(struct cv *cvp, const char *desc);
 void	cv_destroy(struct cv *cvp);
 
 void	cv_wait(struct cv *cvp, struct mtx *mp);
+void	cv_wait_unlock(struct cv *cvp, struct mtx *mp);
 int	cv_wait_sig(struct cv *cvp, struct mtx *mp);
 int	cv_timedwait(struct cv *cvp, struct mtx *mp, int timo);
 int	cv_timedwait_sig(struct cv *cvp, struct mtx *mp, int timo);