Add a new 'show sleepchain' ddb command similar to 'show lockchain' except

that it operates on lockmgr and sx locks.  This can be useful for tracking
down vnode deadlocks in VFS for example.  Note that this command is a bit
more fragile than 'show lockchain' as we have to poke around at the
wait channel of a thread to see if it points to either a struct lock or
a condition variable inside of a struct sx.  If td_wchan points to
something unmapped, then this command will terminate early due to a fault,
but no harm will be done.

1 files changed, 54 insertions, 1 deletions

diff --git a/sys/kern/kern_sx.c b/sys/kern/kern_sx.c
index d215807..fdd8bc9 100644
--- a/sys/kern/kern_sx.c
+++ b/sys/kern/kern_sx.c
@@ -48,9 +48,9 @@ __FBSDID("$FreeBSD$");
 #include <sys/proc.h>
 #include <sys/sx.h>
 
+#ifdef DDB
 #include <ddb/ddb.h>
 
-#ifdef DDB
 static void	db_show_sx(struct lock_object *lock);
 #endif
 
@@ -395,4 +395,57 @@ db_show_sx(struct lock_object *lock)
 	db_printf(" waiters: %d shared, %d exclusive\n", sx->sx_shrd_wcnt,
 	    sx->sx_excl_wcnt);
 }
+
+/*
+ * Check to see if a thread that is blocked on a sleep queue is actually
+ * blocked on an sx lock.  If so, output some details and return true.
+ * If the lock has an exclusive owner, return that in *ownerp.
+ */
+int
+sx_chain(struct thread *td, struct thread **ownerp)
+{
+	struct sx *sx;
+	struct cv *cv;
+
+	/*
+	 * First, see if it looks like td is blocked on a condition
+	 * variable.
+	 */
+	cv = td->td_wchan;
+	if (cv->cv_description != td->td_wmesg)
+		return (0);
+
+	/*
+	 * Ok, see if it looks like td is blocked on the exclusive
+	 * condition variable.
+	 */
+	sx = (struct sx *)((char *)cv - offsetof(struct sx, sx_excl_cv));
+	if (LOCK_CLASS(&sx->sx_object) == &lock_class_sx &&
+	    sx->sx_excl_wcnt > 0)
+		goto ok;
+
+	/*
+	 * Second, see if it looks like td is blocked on the shared
+	 * condition variable.
+	 */
+	sx = (struct sx *)((char *)cv - offsetof(struct sx, sx_shrd_cv));
+	if (LOCK_CLASS(&sx->sx_object) == &lock_class_sx &&
+	    sx->sx_shrd_wcnt > 0)
+		goto ok;
+
+	/* Doesn't seem to be an sx lock. */
+	return (0);
+
+ok:
+	/* We think we have an sx lock, so output some details. */
+	db_printf("blocked on sx \"%s\" ", td->td_wmesg);
+	if (sx->sx_cnt >= 0) {
+		db_printf("SLOCK (count %d)\n", sx->sx_cnt);
+		*ownerp = NULL;
+	} else {
+		db_printf("XLOCK\n");
+		*ownerp = sx->sx_xholder;
+	}
+	return (1);
+}
 #endif