md: fix a warning caused by a race between concurrent md_ioctl()s

Syzkaller reports a warning as belows.

WARNING: CPU: 0 PID: 9647 at drivers/md/md.c:7169
...
Call Trace:
...
RIP: 0010:md_ioctl+0x4017/0x5980 drivers/md/md.c:7169
RSP: 0018:ffff888096027950 EFLAGS: 00010293
RAX: ffff88809322c380 RBX: 0000000000000932 RCX: ffffffff84e266f2
RDX: 0000000000000000 RSI: ffffffff84e299f7 RDI: 0000000000000007
RBP: ffff888096027bc0 R08: ffff88809322c380 R09: ffffed101341a482
R10: ffff888096027940 R11: ffff88809a0d240f R12: 0000000000000932
R13: ffff8880a2c14100 R14: ffff88809a0d2268 R15: ffff88809a0d2408
 __blkdev_driver_ioctl block/ioctl.c:304 [inline]
 blkdev_ioctl+0xece/0x1c10 block/ioctl.c:606
 block_ioctl+0xee/0x130 fs/block_dev.c:1930
 vfs_ioctl fs/ioctl.c:46 [inline]
 file_ioctl fs/ioctl.c:509 [inline]
 do_vfs_ioctl+0xd5f/0x1380 fs/ioctl.c:696
 ksys_ioctl+0xab/0xd0 fs/ioctl.c:713
 __do_sys_ioctl fs/ioctl.c:720 [inline]
 __se_sys_ioctl fs/ioctl.c:718 [inline]
 __x64_sys_ioctl+0x73/0xb0 fs/ioctl.c:718
 do_syscall_64+0xfd/0x680 arch/x86/entry/common.c:301
 entry_SYSCALL_64_after_hwframe+0x49/0xbe

This is caused by a race between two concurrenct md_ioctl()s closing the array.
CPU1 (md_ioctl()) CPU2 (md_ioctl())
------ ------
set_bit(MD_CLOSING, &mddev->flags);

did_set_md_closing = true;
                                    WARN_ON_ONCE(test_bit(MD_CLOSING,
                                            &mddev->flags));
if(did_set_md_closing)
    clear_bit(MD_CLOSING, &mddev->flags);

Fix the warning by returning immediately if the MD_CLOSING bit is set in &mddev->flags which indicates that the array is being closed.

Fixes 065e519e71b2 ("md: MD_CLOSING needs to be cleared after called md_set_readonly or do_md_stop")

Reported-by [email protected]

Signed-off-by Dae R. Jeong <[email protected]>

---
 drivers/md/md.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

drivers/md/md.c +5 -3

index 98bac4f304ae..643f7f5be49b 100644
@@ -7590,8 +7590,11 @@ static int md_ioctl(struct block_device *bdev, fmode_t mode,
                        err = -EBUSY;
                        goto out;
                }
-               WARN_ON_ONCE(test_bit(MD_CLOSING, &mddev->flags));
-               set_bit(MD_CLOSING, &mddev->flags);
+               if (test_and_set_bit(MD_CLOSING, &mddev->flags)) {
+                       mutex_unlock(&mddev->open_mutex);
+                       err = -EBUSY;
+                       goto out;
+               }
                did_set_md_closing = true;
                mutex_unlock(&mddev->open_mutex);
                sync_blockdev(bdev);
--

2.25.1