bdi: use refcount_t for reference counting instead atomic_t

refcount_t type and corresponding API should be used instead of atomic_t
when the variable is used as a reference counter.  This permits avoiding
accidental refcounter overflows that might lead to use-after-free
situations.

Link: http://lkml.kernel.org/r/20180703200141.28415-4-bigeasy@linutronix.de
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 2 insertions, 1 deletions

diff --git a/include/linux/backing-dev-defs.h b/include/linux/backing-dev-defs.h
index 24251762c20c..9a6bc0951cfa 100644
--- a/include/linux/backing-dev-defs.h
+++ b/include/linux/backing-dev-defs.h
@@ -12,6 +12,7 @@
 #include <linux/timer.h>
 #include <linux/workqueue.h>
 #include <linux/kref.h>
+#include <linux/refcount.h>
 
 struct page;
 struct device;
@@ -75,7 +76,7 @@ enum wb_reason {
  */
 struct bdi_writeback_congested {
         unsigned long state;            /* WB_[a]sync_congested flags */
-        atomic_t refcnt;                /* nr of attached wb's and blkg */
+        refcount_t refcnt;              /* nr of attached wb's and blkg */
 
 #ifdef CONFIG_CGROUP_WRITEBACK
         struct backing_dev_info *__bdi; /* the associated bdi, set to NULL