Detect bh refcount underflows.

Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

fs/buffer.c

@@ -46,6 +46,14 @@ static int fsync_buffers_list(spinlock_t *lock, struct list_head *list);
 
 #define BH_ENTRY(list) list_entry((list), struct buffer_head, b_assoc_buffers)
 
+void put_bh(struct buffer_head *bh)
+{
+	WARN_ON(atomic_read(&bh->b_count) <= 0);
+	smp_mb__before_atomic_dec();
+	atomic_dec(&bh->b_count);
+}
+EXPORT_SYMBOL(put_bh);
+
 inline void
 init_buffer(struct buffer_head *bh, bh_end_io_t *handler, void *private)
 {

include/linux/buffer_head.h

@@ -254,11 +254,7 @@ static inline void get_bh(struct buffer_head *bh)
         atomic_inc(&bh->b_count);
 }
 
-static inline void put_bh(struct buffer_head *bh)
-{
-        smp_mb__before_atomic_dec();
-        atomic_dec(&bh->b_count);
-}
+void put_bh(struct buffer_head *bh);
 
 static inline void brelse(struct buffer_head *bh)
 {