bcachefs: Fix race between journal_seq_copy() and journal_seq_drop()

In bch2_btree_interior_update_will_free_node, we copy the journal pins
from outstanding writes on the btree node we're about to free. But, this
can race with the writes completing, and dropping their journal pins.

To guard against this, just use READ_ONCE() in bch2_journal_pin_copy().
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/journal_reclaim.c

@@ -384,12 +384,22 @@ void bch2_journal_pin_set(struct journal *j, u64 seq,
 	struct journal_entry_pin_list *pin_list;
 
 	spin_lock(&j->lock);
+
+	if (seq < journal_last_seq(j)) {
+		/*
+		 * bch2_journal_pin_copy() raced with bch2_journal_pin_drop() on
+		 * the src pin - with the pin dropped, the entry to pin might no
+		 * longer to exist, but that means there's no longer anything to
+		 * copy and we can bail out here:
+		 */
+		spin_unlock(&j->lock);
+		return;
+	}
+
 	pin_list = journal_seq_pin(j, seq);
 
 	__journal_pin_drop(j, pin);
 
-	BUG_ON(!atomic_read(&pin_list->count) && seq == journal_last_seq(j));
-
 	atomic_inc(&pin_list->count);
 	pin->seq	= seq;
 	pin->flush	= flush_fn;

fs/bcachefs/journal_reclaim.h

@@ -53,8 +53,11 @@ static inline void bch2_journal_pin_copy(struct journal *j,
 					 struct journal_entry_pin *src,
 					 journal_pin_flush_fn flush_fn)
 {
-	if (journal_pin_active(src))
-		bch2_journal_pin_add(j, src->seq, dst, flush_fn);
+	/* Guard against racing with journal_pin_drop(src): */
+	u64 seq = READ_ONCE(src->seq);
+
+	if (seq)
+		bch2_journal_pin_add(j, seq, dst, flush_fn);
 }
 
 static inline void bch2_journal_pin_update(struct journal *j, u64 seq,