bcachefs: Fix race leading to btree node write getting stuck

Checking btree_node_may_write() isn't atomic with the other btree flags,
dirty and need_write in particular. There was a rare race where we'd
unblock a node from writing while __btree_node_flush() was setting
need_write, and no thread would notice that the node was now both able
to write and needed to be written.

Fix this by adding btree node flags for will_make_reachable and
write_blocked that can be checked in the cmpxchg loop in
__bch2_btree_node_write.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>

fs/bcachefs/btree_cache.c

@@ -223,10 +223,9 @@ static int __btree_node_reclaim(struct bch_fs *c, struct btree *b, bool flush)
 		goto wait_on_io;
 	}
 
-	if (btree_node_noevict(b))
-		goto out_unlock;
-
-	if (!btree_node_may_write(b))
+	if (btree_node_noevict(b) ||
+	    btree_node_write_blocked(b) ||
+	    btree_node_will_make_reachable(b))
 		goto out_unlock;
 
 	if (btree_node_dirty(b)) {

fs/bcachefs/btree_io.c

@@ -1606,7 +1606,8 @@ static void __btree_node_write_done(struct bch_fs *c, struct btree *b)
 		if ((old & (1U << BTREE_NODE_dirty)) &&
 		    (old & (1U << BTREE_NODE_need_write)) &&
 		    !(old & (1U << BTREE_NODE_never_write)) &&
-		    btree_node_may_write(b)) {
+		    !(old & (1U << BTREE_NODE_write_blocked)) &&
+		    !(old & (1U << BTREE_NODE_will_make_reachable))) {
 			new &= ~(1U << BTREE_NODE_dirty);
 			new &= ~(1U << BTREE_NODE_need_write);
 			new |=  (1U << BTREE_NODE_write_in_flight);
@@ -1778,10 +1779,13 @@ void __bch2_btree_node_write(struct bch_fs *c, struct btree *b, unsigned flags)
 		    !(old & (1 << BTREE_NODE_need_write)))
 			return;
 
-		if (!btree_node_may_write(b))
+		if (old &
+		    ((1 << BTREE_NODE_never_write)|
+		     (1 << BTREE_NODE_write_blocked)))
 			return;
 
-		if (old & (1 << BTREE_NODE_never_write))
+		if (b->written &&
+		    (old & (1 << BTREE_NODE_will_make_reachable)))
 			return;
 
 		if (old & (1 << BTREE_NODE_write_in_flight))

fs/bcachefs/btree_io.h

@@ -62,12 +62,6 @@ void __bch2_btree_node_wait_on_write(struct btree *);
 void bch2_btree_node_wait_on_read(struct btree *);
 void bch2_btree_node_wait_on_write(struct btree *);
 
-static inline bool btree_node_may_write(struct btree *b)
-{
-	return list_empty_careful(&b->write_blocked) &&
-		(!b->written || !b->will_make_reachable);
-}
-
 enum compact_mode {
 	COMPACT_LAZY,
 	COMPACT_ALL,

fs/bcachefs/btree_types.h

@@ -434,6 +434,8 @@ struct btree_trans {
 	x(read_error)							\
 	x(dirty)							\
 	x(need_write)							\
+	x(write_blocked)						\
+	x(will_make_reachable)						\
 	x(noevict)							\
 	x(write_idx)							\
 	x(accessed)							\

fs/bcachefs/btree_update_interior.c

@@ -606,6 +606,8 @@ static void btree_update_nodes_written(struct btree_update *as)
 		mutex_lock(&c->btree_interior_update_lock);
 
 		list_del(&as->write_blocked_list);
+		if (list_empty(&b->write_blocked))
+			clear_btree_node_write_blocked(b);
 
 		/*
 		 * Node might have been freed, recheck under
@@ -650,6 +652,7 @@ static void btree_update_nodes_written(struct btree_update *as)
 
 		BUG_ON(b->will_make_reachable != (unsigned long) as);
 		b->will_make_reachable = 0;
+		clear_btree_node_will_make_reachable(b);
 	}
 	mutex_unlock(&c->btree_interior_update_lock);
 
@@ -716,6 +719,8 @@ static void btree_update_updated_node(struct btree_update *as, struct btree *b)
 
 	as->mode	= BTREE_INTERIOR_UPDATING_NODE;
 	as->b		= b;
+
+	set_btree_node_write_blocked(b);
 	list_add(&as->write_blocked_list, &b->write_blocked);
 
 	mutex_unlock(&c->btree_interior_update_lock);
@@ -781,6 +786,7 @@ static void bch2_btree_update_add_new_node(struct btree_update *as, struct btree
 
 	as->new_nodes[as->nr_new_nodes++] = b;
 	b->will_make_reachable = 1UL|(unsigned long) as;
+	set_btree_node_will_make_reachable(b);
 
 	mutex_unlock(&c->btree_interior_update_lock);
 
@@ -803,6 +809,7 @@ static void btree_update_drop_new_node(struct bch_fs *c, struct btree *b)
 	 * xchg() is for synchronization with bch2_btree_complete_write:
 	 */
 	v = xchg(&b->will_make_reachable, 0);
+	clear_btree_node_will_make_reachable(b);
 	as = (struct btree_update *) (v & ~1UL);
 
 	if (!as) {