md: fix regression for null-ptr-deference in __md_stop()

Commit 3e453522 ("md: Free resources in __md_stop") tried to fix
null-ptr-deference for 'active_io' by moving percpu_ref_exit() to
__md_stop(), however, the commit also moving 'writes_pending' to
__md_stop(), and this will cause mdadm tests broken:

BUG: kernel NULL pointer dereference, address: 0000000000000038
Oops: 0000 [#1] PREEMPT SMP
CPU: 15 PID: 17830 Comm: mdadm Not tainted 6.3.0-rc3-next-20230324-00009-g520d37
RIP: 0010:free_percpu+0x465/0x670
Call Trace:
 <TASK>
 __percpu_ref_exit+0x48/0x70
 percpu_ref_exit+0x1a/0x90
 __md_stop+0xe9/0x170
 do_md_stop+0x1e1/0x7b0
 md_ioctl+0x90c/0x1aa0
 blkdev_ioctl+0x19b/0x400
 vfs_ioctl+0x20/0x50
 __x64_sys_ioctl+0xba/0xe0
 do_syscall_64+0x6c/0xe0
 entry_SYSCALL_64_after_hwframe+0x63/0xcd

And the problem can be reporduced 100% by following test:

mdadm -CR /dev/md0 -l1 -n1 /dev/sda --force
echo inactive > /sys/block/md0/md/array_state
echo read-auto  > /sys/block/md0/md/array_state
echo inactive > /sys/block/md0/md/array_state

Root cause:

// start raid
raid1_run
 mddev_init_writes_pending
  percpu_ref_init

// inactive raid
array_state_store
 do_md_stop
  __md_stop
   percpu_ref_exit

// start raid again
array_state_store
 do_md_run
  raid1_run
   mddev_init_writes_pending
    if (mddev->writes_pending.percpu_count_ptr)
    // won't reinit

// inactive raid again
...
percpu_ref_exit
-> null-ptr-deference

Before the commit, 'writes_pending' is exited when mddev is freed, and
it's safe to restart raid because mddev_init_writes_pending() already make
sure that 'writes_pending' will only be initialized once.

Fix the prblem by moving 'writes_pending' back, it's a litter hard to find
the relationship between alloc memory and free memory, however, code
changes is much less and we lived with this for a long time already.

Fixes: 3e453522 ("md: Free resources in __md_stop")
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Xiao Ni <xni@redhat.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20230328094400.1448955-1-yukuai1@huaweicloud.com

drivers/md/md.c

@@ -6260,7 +6260,6 @@ static void __md_stop(struct mddev *mddev)
 	module_put(pers->owner);
 	clear_bit(MD_RECOVERY_FROZEN, &mddev->recovery);
 
-	percpu_ref_exit(&mddev->writes_pending);
 	percpu_ref_exit(&mddev->active_io);
 	bioset_exit(&mddev->bio_set);
 	bioset_exit(&mddev->sync_set);
@@ -6273,6 +6272,7 @@ void md_stop(struct mddev *mddev)
 	 */
 	__md_stop_writes(mddev);
 	__md_stop(mddev);
+	percpu_ref_exit(&mddev->writes_pending);
 }
 
 EXPORT_SYMBOL_GPL(md_stop);
@@ -7843,6 +7843,7 @@ static void md_free_disk(struct gendisk *disk)
 {
 	struct mddev *mddev = disk->private_data;
 
+	percpu_ref_exit(&mddev->writes_pending);
 	mddev_free(mddev);
 }