md: fix a crash in mempool_free

There's a crash in mempool_free when running the lvm test
shell/lvchange-rebuild-raid.sh.

The reason for the crash is this:
* super_written calls atomic_dec_and_test(&mddev->pending_writes) and
  wake_up(&mddev->sb_wait). Then it calls rdev_dec_pending(rdev, mddev)
  and bio_put(bio).
* so, the process that waited on sb_wait and that is woken up is racing
  with bio_put(bio).
* if the process wins the race, it calls bioset_exit before bio_put(bio)
  is executed.
* bio_put(bio) attempts to free a bio into a destroyed bio set - causing
  a crash in mempool_free.

We fix this bug by moving bio_put before atomic_dec_and_test.

We also move rdev_dec_pending before atomic_dec_and_test as suggested by
Neil Brown.

The function md_end_flush has a similar bug - we must call bio_put before
we decrement the number of in-progress bios.

 BUG: kernel NULL pointer dereference, address: 0000000000000000
 #PF: supervisor write access in kernel mode
 #PF: error_code(0x0002) - not-present page
 PGD 11557f0067 P4D 11557f0067 PUD 0
 Oops: 0002 [#1] PREEMPT SMP
 CPU: 0 PID: 73 Comm: kworker/0:1 Not tainted 6.1.0-rc3 #5
 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-2 04/01/2014
 Workqueue: kdelayd flush_expired_bios [dm_delay]
 RIP: 0010:mempool_free+0x47/0x80
 Code: 48 89 ef 5b 5d ff e0 f3 c3 48 89 f7 e8 32 45 3f 00 48 63 53 08 48 89 c6 3b 53 04 7d 2d 48 8b 43 10 8d 4a 01 48 89 df 89 4b 08 <48> 89 2c d0 e8 b0 45 3f 00 48 8d 7b 30 5b 5d 31 c9 ba 01 00 00 00
 RSP: 0018:ffff88910036bda8 EFLAGS: 00010093
 RAX: 0000000000000000 RBX: ffff8891037b65d8 RCX: 0000000000000001
 RDX: 0000000000000000 RSI: 0000000000000202 RDI: ffff8891037b65d8
 RBP: ffff8891447ba240 R08: 0000000000012908 R09: 00000000003d0900
 R10: 0000000000000000 R11: 0000000000173544 R12: ffff889101a14000
 R13: ffff8891562ac300 R14: ffff889102b41440 R15: ffffe8ffffa00d05
 FS:  0000000000000000(0000) GS:ffff88942fa00000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 0000000000000000 CR3: 0000001102e99000 CR4: 00000000000006b0
 Call Trace:
  <TASK>
  clone_endio+0xf4/0x1c0 [dm_mod]
  clone_endio+0xf4/0x1c0 [dm_mod]
  __submit_bio+0x76/0x120
  submit_bio_noacct_nocheck+0xb6/0x2a0
  flush_expired_bios+0x28/0x2f [dm_delay]
  process_one_work+0x1b4/0x300
  worker_thread+0x45/0x3e0
  ? rescuer_thread+0x380/0x380
  kthread+0xc2/0x100
  ? kthread_complete_and_exit+0x20/0x20
  ret_from_fork+0x1f/0x30
  </TASK>
 Modules linked in: brd dm_delay dm_raid dm_mod af_packet uvesafb cfbfillrect cfbimgblt cn cfbcopyarea fb font fbdev tun autofs4 binfmt_misc configfs ipv6 virtio_rng virtio_balloon rng_core virtio_net pcspkr net_failover failover qemu_fw_cfg button mousedev raid10 raid456 libcrc32c async_raid6_recov async_memcpy async_pq raid6_pq async_xor xor async_tx raid1 raid0 md_mod sd_mod t10_pi crc64_rocksoft crc64 virtio_scsi scsi_mod evdev psmouse bsg scsi_common [last unloaded: brd]
 CR2: 0000000000000000
 ---[ end trace 0000000000000000 ]---
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Song Liu <song@kernel.org>

drivers/md/md.c

@@ -521,13 +521,14 @@ static void md_end_flush(struct bio *bio)
 	struct md_rdev *rdev = bio->bi_private;
 	struct mddev *mddev = rdev->mddev;
 
+	bio_put(bio);
+
 	rdev_dec_pending(rdev, mddev);
 
 	if (atomic_dec_and_test(&mddev->flush_pending)) {
 		/* The pre-request flush has finished */
 		queue_work(md_wq, &mddev->flush_work);
 	}
-	bio_put(bio);
 }
 
 static void md_submit_flush_data(struct work_struct *ws);
@@ -925,10 +926,12 @@ static void super_written(struct bio *bio)
 	} else
 		clear_bit(LastDev, &rdev->flags);
 
+	bio_put(bio);
+
+	rdev_dec_pending(rdev, mddev);
+
 	if (atomic_dec_and_test(&mddev->pending_writes))
 		wake_up(&mddev->sb_wait);
-	rdev_dec_pending(rdev, mddev);
-	bio_put(bio);
 }
 
 void md_super_write(struct mddev *mddev, struct md_rdev *rdev,