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>

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

include/linux/backing-dev.h

@@ -404,13 +404,13 @@ static inline bool inode_cgwb_enabled(struct inode *inode)
 static inline struct bdi_writeback_congested *
 wb_congested_get_create(struct backing_dev_info *bdi, int blkcg_id, gfp_t gfp)
 {
-	atomic_inc(&bdi->wb_congested->refcnt);
+	refcount_inc(&bdi->wb_congested->refcnt);
 	return bdi->wb_congested;
 }
 
 static inline void wb_congested_put(struct bdi_writeback_congested *congested)
 {
-	if (atomic_dec_and_test(&congested->refcnt))
+	if (refcount_dec_and_test(&congested->refcnt))
 		kfree(congested);
 }
 

mm/backing-dev.c

@@ -438,10 +438,10 @@ retry:
 	if (new_congested) {
 		/* !found and storage for new one already allocated, insert */
 		congested = new_congested;
-		new_congested = NULL;
 		rb_link_node(&congested->rb_node, parent, node);
 		rb_insert_color(&congested->rb_node, &bdi->cgwb_congested_tree);
-		goto found;
+		spin_unlock_irqrestore(&cgwb_lock, flags);
+		return congested;
 	}
 
 	spin_unlock_irqrestore(&cgwb_lock, flags);
@@ -451,13 +451,13 @@ retry:
 	if (!new_congested)
 		return NULL;
 
-	atomic_set(&new_congested->refcnt, 0);
+	refcount_set(&new_congested->refcnt, 1);
 	new_congested->__bdi = bdi;
 	new_congested->blkcg_id = blkcg_id;
 	goto retry;
 
 found:
-	atomic_inc(&congested->refcnt);
+	refcount_inc(&congested->refcnt);
 	spin_unlock_irqrestore(&cgwb_lock, flags);
 	kfree(new_congested);
 	return congested;
@@ -474,7 +474,7 @@ void wb_congested_put(struct bdi_writeback_congested *congested)
 	unsigned long flags;
 
 	local_irq_save(flags);
-	if (!atomic_dec_and_lock(&congested->refcnt, &cgwb_lock)) {
+	if (!refcount_dec_and_lock(&congested->refcnt, &cgwb_lock)) {
 		local_irq_restore(flags);
 		return;
 	}
@@ -804,7 +804,7 @@ static int cgwb_bdi_init(struct backing_dev_info *bdi)
 	if (!bdi->wb_congested)
 		return -ENOMEM;
 
-	atomic_set(&bdi->wb_congested->refcnt, 1);
+	refcount_set(&bdi->wb_congested->refcnt, 1);
 
 	err = wb_init(&bdi->wb, bdi, 1, GFP_KERNEL);
 	if (err) {