erofs: initialized fields can only be observed after bit is set

Currently, although set_bit() & test_bit() pairs are used as a fast-
path for initialized configurations. However, these atomic ops are
actually relaxed forms. Instead, load-acquire & store-release form is
needed to make sure uninitialized fields won't be observed in advance
here (yet no such corresponding bitops so use full barriers instead.)

Link: https://lore.kernel.org/r/20210209130618.15838-1-hsiangkao@aol.com
Fixes: 62dc4597 ("staging: erofs: fix race of initializing xattrs of a inode at the same time")
Fixes: 152a333a ("staging: erofs: add compacted compression indexes support")
Cc: <stable@vger.kernel.org> # 5.3+
Reported-by: Huang Jianan <huangjianan@oppo.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <hsiangkao@redhat.com>

fs/erofs/xattr.c

@@ -48,8 +48,14 @@ static int init_inode_xattrs(struct inode *inode)
 	int ret = 0;
 
 	/* the most case is that xattrs of this inode are initialized. */
-	if (test_bit(EROFS_I_EA_INITED_BIT, &vi->flags))
+	if (test_bit(EROFS_I_EA_INITED_BIT, &vi->flags)) {
+		/*
+		 * paired with smp_mb() at the end of the function to ensure
+		 * fields will only be observed after the bit is set.
+		 */
+		smp_mb();
 		return 0;
+	}
 
 	if (wait_on_bit_lock(&vi->flags, EROFS_I_BL_XATTR_BIT, TASK_KILLABLE))
 		return -ERESTARTSYS;
@@ -137,6 +143,8 @@ static int init_inode_xattrs(struct inode *inode)
 	}
 	xattr_iter_end(&it, atomic_map);
 
+	/* paired with smp_mb() at the beginning of the function. */
+	smp_mb();
 	set_bit(EROFS_I_EA_INITED_BIT, &vi->flags);
 
 out_unlock:

fs/erofs/zmap.c

@@ -36,8 +36,14 @@ static int z_erofs_fill_inode_lazy(struct inode *inode)
 	void *kaddr;
 	struct z_erofs_map_header *h;
 
-	if (test_bit(EROFS_I_Z_INITED_BIT, &vi->flags))
+	if (test_bit(EROFS_I_Z_INITED_BIT, &vi->flags)) {
+		/*
+		 * paired with smp_mb() at the end of the function to ensure
+		 * fields will only be observed after the bit is set.
+		 */
+		smp_mb();
 		return 0;
+	}
 
 	if (wait_on_bit_lock(&vi->flags, EROFS_I_BL_Z_BIT, TASK_KILLABLE))
 		return -ERESTARTSYS;
@@ -83,6 +89,8 @@ static int z_erofs_fill_inode_lazy(struct inode *inode)
 
 	vi->z_physical_clusterbits[1] = vi->z_logical_clusterbits +
 					((h->h_clusterbits >> 5) & 7);
+	/* paired with smp_mb() at the beginning of the function */
+	smp_mb();
 	set_bit(EROFS_I_Z_INITED_BIT, &vi->flags);
 unmap_done:
 	kunmap_atomic(kaddr);