[PATCH 3/6] bdi: Use refcount_t for reference counting instead atomic_t

[Sebastian Andrzej Siewior]

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

Cc: linux...@kvack.org
Cc: Andrew Morton <a...@linux-foundation.org>
Suggested-by: Peter Zijlstra <pet...@infradead.org>
Acked-by: Peter Zijlstra (Intel) <pet...@infradead.org>
Signed-off-by: Sebastian Andrzej Siewior <bige...@linutronix.de>
---
 include/linux/backing-dev-defs.h |  3 ++-
 include/linux/backing-dev.h      |  4 ++--
 mm/backing-dev.c                 | 12 ++++++------
 3 files changed, 10 insertions(+), 9 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
diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h
index 72ca0f3d39f3..c28a47cbe355 100644
--- a/include/linux/backing-dev.h
+++ b/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);
 }
 
diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 2e5d3df0853d..55a233d75f39 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -438,10 +438,10 @@ wb_congested_get_create(struct backing_dev_info *bdi, int 
blkcg_id, gfp_t gfp)
        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 @@ wb_congested_get_create(struct backing_dev_info *bdi, int 
blkcg_id, gfp_t gfp)
        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) {
-- 
2.18.0