fs, nfs: convert pnfs_layout_segment.pls_refcount from atomic_t to refcount_t

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.

Signed-off-by: Elena Reshetova <[e-mail]>
Signed-off-by: Hans Liljestrand <[e-mail]>
Signed-off-by: Kees Cook <[e-mail]>
Signed-off-by: David Windsor <[e-mail]>
Signed-off-by: Anna Schumaker <[e-mail]>

fs/nfs/pnfs.c

···
 {
 	INIT_LIST_HEAD(&lseg->pls_list);
 	INIT_LIST_HEAD(&lseg->pls_lc_list);
-	atomic_set(&lseg->pls_refcount, 1);
+	refcount_set(&lseg->pls_refcount, 1);
 	set_bit(NFS_LSEG_VALID, &lseg->pls_flags);
 	lseg->pls_layout = lo;
 	lseg->pls_range = *range;
···
 		return;
 
 	dprintk("%s: lseg %p ref %d valid %d\n", __func__, lseg,
-		atomic_read(&lseg->pls_refcount),
+		refcount_read(&lseg->pls_refcount),
 		test_bit(NFS_LSEG_VALID, &lseg->pls_flags));
 
 	lo = lseg->pls_layout;
 	inode = lo->plh_inode;
 
-	if (atomic_dec_and_lock(&lseg->pls_refcount, &inode->i_lock)) {
+	if (refcount_dec_and_lock(&lseg->pls_refcount, &inode->i_lock)) {
 		if (test_bit(NFS_LSEG_VALID, &lseg->pls_flags)) {
 			spin_unlock(&inode->i_lock);
 			return;
···
 static bool pnfs_lseg_dec_and_remove_zero(struct pnfs_layout_segment *lseg,
 		struct list_head *tmp_list)
 {
-	if (!atomic_dec_and_test(&lseg->pls_refcount))
+	if (!refcount_dec_and_test(&lseg->pls_refcount))
 		return false;
 	pnfs_layout_remove_lseg(lseg->pls_layout, lseg);
 	list_add(&lseg->pls_list, tmp_list);
···
 		 * outstanding io is finished.
 		 */
 		dprintk("%s: lseg %p ref %d\n", __func__, lseg,
-			atomic_read(&lseg->pls_refcount));
+			refcount_read(&lseg->pls_refcount));
 		if (pnfs_lseg_dec_and_remove_zero(lseg, tmp_list))
 			rv = 1;
 	}
···
 	}
 
 	dprintk("%s:Return lseg %p ref %d\n",
-		__func__, ret, ret ? atomic_read(&ret->pls_refcount) : 0);
+		__func__, ret, ret ? refcount_read(&ret->pls_refcount) : 0);
 	return ret;
 }
 

fs/nfs/pnfs.h

···
 	struct list_head pls_list;
 	struct list_head pls_lc_list;
 	struct pnfs_layout_range pls_range;
-	atomic_t pls_refcount;
+	refcount_t pls_refcount;
 	u32 pls_seq;
 	unsigned long pls_flags;
 	struct pnfs_layout_hdr *pls_layout;
···
 pnfs_get_lseg(struct pnfs_layout_segment *lseg)
 {
 	if (lseg) {
-		atomic_inc(&lseg->pls_refcount);
+		refcount_inc(&lseg->pls_refcount);
 		smp_mb__after_atomic();
 	}
 	return lseg;