closures: fix closure_sync + closure debugging

originally, stack closures were only used synchronously, and with the
original implementation of closure_sync() the ref never hit 0; thus,
closure_put_after_sub() assumes that if the ref hits 0 it's on the debug
list, in debug mode.

that's no longer true with the current implementation of closure_sync,
so we need a new magic so closure_debug_destroy() doesn't pop an assert.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

include/linux/closure.h

@@ -159,6 +159,7 @@ struct closure {
 #ifdef CONFIG_DEBUG_CLOSURES
 #define CLOSURE_MAGIC_DEAD	0xc054dead
 #define CLOSURE_MAGIC_ALIVE	0xc054a11e
+#define CLOSURE_MAGIC_STACK	0xc05451cc
 
 	unsigned int		magic;
 	struct list_head	all;
@@ -323,12 +324,18 @@ static inline void closure_init_stack(struct closure *cl)
 {
 	memset(cl, 0, sizeof(struct closure));
 	atomic_set(&cl->remaining, CLOSURE_REMAINING_INITIALIZER);
+#ifdef CONFIG_DEBUG_CLOSURES
+	cl->magic = CLOSURE_MAGIC_STACK;
+#endif
 }
 
 static inline void closure_init_stack_release(struct closure *cl)
 {
 	memset(cl, 0, sizeof(struct closure));
 	atomic_set_release(&cl->remaining, CLOSURE_REMAINING_INITIALIZER);
+#ifdef CONFIG_DEBUG_CLOSURES
+	cl->magic = CLOSURE_MAGIC_STACK;
+#endif
 }
 
 /**

lib/closure.c

@@ -244,6 +244,9 @@ void closure_debug_destroy(struct closure *cl)
 {
 	unsigned long flags;
 
+	if (cl->magic == CLOSURE_MAGIC_STACK)
+		return;
+
 	BUG_ON(cl->magic != CLOSURE_MAGIC_ALIVE);
 	cl->magic = CLOSURE_MAGIC_DEAD;