[PATCH] dm: fix false alarm in free_rq_clone() for non blk-mq target

[Junichi Nomura <j-nomura@ce.jp.nec.com>]

When stacking request-based dm device on non blk-mq device
and device-mapper target could not map the request
(error target is used, multipath target with all paths down, etc),
following warning will show up once:

  WARNING: CPU: 7 PID: 0 at drivers/md/dm.c:1090 free_rq_clone+0x7a/0xf0
  CPU: 7 PID: 0 Comm: swapper/7 Not tainted 4.1.0-rc5+ #1
  ..
  Call Trace:
   <IRQ>  [<ffffffff81558095>] dump_stack+0x45/0x57
   [<ffffffff8105ef3a>] warn_slowpath_common+0x8a/0xc0
   [<ffffffff8105f02a>] warn_slowpath_null+0x1a/0x20
   [<ffffffffa020bf3a>] free_rq_clone+0x7a/0xf0 [dm_mod]
   [<ffffffffa020c7cc>] dm_softirq_done+0x13c/0x250 [dm_mod]
   [<ffffffff812b87a0>] blk_done_softirq+0x80/0xa0
   [<ffffffff81062f44>] __do_softirq+0xf4/0x2d0
   [<ffffffff81063425>] irq_exit+0x125/0x130
   [<ffffffff81037f05>] smp_call_function_single_interrupt+0x35/0x40
   [<ffffffff8156027e>] call_function_single_interrupt+0x6e/0x80
   <EOI>  [<ffffffff8104b576>] ? native_safe_halt+0x6/0x10
   [<ffffffff810b6348>] ? rcu_eqs_enter+0x68/0x90
   [<ffffffff8100d6fe>] default_idle+0x1e/0xc0
   [<ffffffff8100e1df>] arch_cpu_idle+0xf/0x20
   [<ffffffff810a269c>] cpu_startup_entry+0x2fc/0x3a0
   [<ffffffff81038892>] start_secondary+0x182/0x1b0

The warning was added by commit aa6df8dd28c0 ("dm: fix free_rq_clone()
NULL pointer when requeueing unmapped request").

But free_rq_clone() with clone->q == NULL is valid usage for non blk-mq
underlying device. Such a call can happen via dm_kill_unmapped_request().

Signed-off-by: Jun'ichi Nomura <j-nomura@ce.jp.nec.com>

diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index 1badfb2..8ecd22b 100644
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -1087,14 +1087,13 @@ static void free_rq_clone(struct request *clone, bool must_be_mapped)
 	struct dm_rq_target_io *tio = clone->end_io_data;
 	struct mapped_device *md = tio->md;
 
-	WARN_ON_ONCE(must_be_mapped && !clone->q);
-
 	blk_rq_unprep_clone(clone);
 
-	if (md->type == DM_TYPE_MQ_REQUEST_BASED)
+	if (md->type == DM_TYPE_MQ_REQUEST_BASED) {
 		/* stacked on blk-mq queue(s) */
+		WARN_ON_ONCE(must_be_mapped && !clone->q);
 		tio->ti->type->release_clone_rq(clone);
-	else if (!md->queue->mq_ops)
+	} else if (!md->queue->mq_ops)
 		/* request_fn queue stacked on request_fn queue(s) */
 		free_clone_request(md, clone);
 	/*