Permit Giant to be passed as the explicit interlock either to

msleep/mtx_sleep or the various cv_*wait*() routines.  Currently, the
"unlock" behavior of PDROP and cv_wait_unlock() with Giant is not
permitted as it is will be confusing since Giant is fully unrecursed and
unlocked during a thread sleep.

This is handy for subsystems which wish to allow unlocked drivers to
continue to use Giant such as CAM, the new TTY layer, and the new USB
stack.  CAM currently uses a hack that I told Scott to use because I
really didn't want to permit this behavior, and the TTY and USB patches
both have various patches to permit this.

MFC after:	2 weeks

1 files changed, 6 insertions, 2 deletions

diff --git a/sys/kern/kern_synch.c b/sys/kern/kern_synch.c
index 6c10ba4..ef682f4 100644
--- a/sys/kern/kern_synch.c
+++ b/sys/kern/kern_synch.c
@@ -140,6 +140,9 @@ _sleep(void *ident, struct lock_object *lock, int priority,
 	    ident == &lbolt, ("sleeping without a lock"));
 	KASSERT(p != NULL, ("msleep1"));
 	KASSERT(ident != NULL && TD_IS_RUNNING(td), ("msleep"));
+	if (priority & PDROP)
+		KASSERT(lock != NULL && lock != &Giant.lock_object,
+		    ("PDROP requires a non-Giant lock"));
 	if (lock != NULL)
 		class = LOCK_CLASS(lock);
 	else
@@ -182,7 +185,8 @@ _sleep(void *ident, struct lock_object *lock, int priority,
 	    td->td_tid, p->p_pid, td->td_name, wmesg, ident);
 
 	DROP_GIANT();
-	if (lock != NULL && !(class->lc_flags & LC_SLEEPABLE)) {
+	if (lock != NULL && lock != &Giant.lock_object &&
+	    !(class->lc_flags & LC_SLEEPABLE)) {
 		WITNESS_SAVE(lock, lock_witness);
 		lock_state = class->lc_unlock(lock);
 	} else
@@ -222,7 +226,7 @@ _sleep(void *ident, struct lock_object *lock, int priority,
 		ktrcsw(0, 0);
 #endif
 	PICKUP_GIANT();
-	if (lock != NULL && !(priority & PDROP)) {
+	if (lock != NULL && lock != &Giant.lock_object && !(priority & PDROP)) {
 		class->lc_lock(lock, lock_state);
 		WITNESS_RESTORE(lock, lock_witness);
 	}