md/bitmap: wait for external bitmap writes to complete during tear down

NULL pointer dereference was observed in super_written() when it tries
to access the mddev structure.

[The below stack trace is from an older kernel, but the problem described
in this patch applies to the mainline kernel.]

[ 1194.474861] task: ffff8fdd20858000 task.stack: ffffb99d40790000
[ 1194.488000] RIP: 0010:super_written+0x29/0xe1
[ 1194.499688] RSP: 0018:ffff8ffb7fcc3c78 EFLAGS: 00010046
[ 1194.512477] RAX: 0000000000000000 RBX: ffff8ffb7bf4a000 RCX: ffff8ffb78991048
[ 1194.527325] RDX: 0000000000000001 RSI: 0000000000000000 RDI: ffff8ffb56b8a200
[ 1194.542576] RBP: ffff8ffb7fcc3c90 R08: 000000000000000b R09: 0000000000000000
[ 1194.558001] R10: ffff8ffb56b8a298 R11: 0000000000000000 R12: ffff8ffb56b8a200
[ 1194.573070] R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
[ 1194.588117] FS:  0000000000000000(0000) GS:ffff8ffb7fcc0000(0000) knlGS:0000000000000000
[ 1194.604264] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 1194.617375] CR2: 00000000000002b8 CR3: 00000021e040a002 CR4: 00000000007606e0
[ 1194.632327] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 1194.647865] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[ 1194.663316] PKRU: 55555554
[ 1194.674090] Call Trace:
[ 1194.683735]  <IRQ>
[ 1194.692948]  bio_endio+0xae/0x135
[ 1194.703580]  blk_update_request+0xad/0x2fa
[ 1194.714990]  blk_update_bidi_request+0x20/0x72
[ 1194.726578]  __blk_end_bidi_request+0x2c/0x4d
[ 1194.738373]  __blk_end_request_all+0x31/0x49
[ 1194.749344]  blk_flush_complete_seq+0x377/0x383
[ 1194.761550]  flush_end_io+0x1dd/0x2a7
[ 1194.772910]  blk_finish_request+0x9f/0x13c
[ 1194.784544]  scsi_end_request+0x180/0x25c
[ 1194.796149]  scsi_io_completion+0xc8/0x610
[ 1194.807503]  scsi_finish_command+0xdc/0x125
[ 1194.818897]  scsi_softirq_done+0x81/0xde
[ 1194.830062]  blk_done_softirq+0xa4/0xcc
[ 1194.841008]  __do_softirq+0xd9/0x29f
[ 1194.851257]  irq_exit+0xe6/0xeb
[ 1194.861290]  do_IRQ+0x59/0xe3
[ 1194.871060]  common_interrupt+0x1c6/0x382
[ 1194.881988]  </IRQ>
[ 1194.890646] RIP: 0010:cpuidle_enter_state+0xdd/0x2a5
[ 1194.902532] RSP: 0018:ffffb99d40793e68 EFLAGS: 00000246 ORIG_RAX: ffffffffffffff43
[ 1194.917317] RAX: ffff8ffb7fce27c0 RBX: ffff8ffb7fced800 RCX: 000000000000001f
[ 1194.932056] RDX: 0000000000000000 RSI: 0000000000000004 RDI: 0000000000000000
[ 1194.946428] RBP: ffffb99d40793ea0 R08: 0000000000000004 R09: 0000000000002ed2
[ 1194.960508] R10: 0000000000002664 R11: 0000000000000018 R12: 0000000000000003
[ 1194.974454] R13: 000000000000000b R14: ffffffff925715a0 R15: 0000011610120d5a
[ 1194.988607]  ? cpuidle_enter_state+0xcc/0x2a5
[ 1194.999077]  cpuidle_enter+0x17/0x19
[ 1195.008395]  call_cpuidle+0x23/0x3a
[ 1195.017718]  do_idle+0x172/0x1d5
[ 1195.026358]  cpu_startup_entry+0x73/0x75
[ 1195.035769]  start_secondary+0x1b9/0x20b
[ 1195.044894]  secondary_startup_64+0xa5/0xa5
[ 1195.084921] RIP: super_written+0x29/0xe1 RSP: ffff8ffb7fcc3c78
[ 1195.096354] CR2: 00000000000002b8

bio in the above stack is a bitmap write whose completion is invoked after
the tear down sequence sets the mddev structure to NULL in rdev.

During tear down, there is an attempt to flush the bitmap writes, but for
external bitmaps, there is no explicit wait for all the bitmap writes to
complete. For instance, md_bitmap_flush() is called to flush the bitmap
writes, but the last call to md_bitmap_daemon_work() in md_bitmap_flush()
could generate new bitmap writes for which there is no explicit wait to
complete those writes. The call to md_bitmap_update_sb() will return
simply for external bitmaps and the follow-up call to md_update_sb() is
conditional and may not get called for external bitmaps. This results in a
kernel panic when the completion routine, super_written() is called which
tries to reference mddev in the rdev that has been set to
NULL(in unbind_rdev_from_array() by tear down sequence).

The solution is to call md_super_wait() for external bitmaps after the
last call to md_bitmap_daemon_work() in md_bitmap_flush() to ensure there
are no pending bitmap writes before proceeding with the tear down.

Cc: stable@vger.kernel.org
Signed-off-by: Sudhakar Panneerselvam <sudhakar.panneerselvam@oracle.com>
Reviewed-by: Zhao Heming <heming.zhao@suse.com>
Signed-off-by: Song Liu <song@kernel.org>

drivers/md/md-bitmap.c

@@ -1722,6 +1722,8 @@ void md_bitmap_flush(struct mddev *mddev)
 	md_bitmap_daemon_work(mddev);
 	bitmap->daemon_lastrun -= sleep;
 	md_bitmap_daemon_work(mddev);
+	if (mddev->bitmap_info.external)
+		md_super_wait(mddev);
 	md_bitmap_update_sb(bitmap);
 }