drm/dp/mst: drop cancel work sync in the mstb destroy path (v2)

Since 9eb1e57f564d4e6e10991402726cc83fe0b9172f
drm/dp/mst: make sure mst_primary mstb is valid in work function

we validate the mstb structs in the work function, and doing
that takes a reference. So we should never get here with the
work function running using the mstb device, only if the work
function hasn't run yet or is running for another mstb.

So we don't need to sync the work here, this was causing
lockdep spew as below.

[ +0.000160] =============================================
[ +0.000001] [ INFO: possible recursive locking detected ]
[ +0.000002] 3.10.0-320.el7.rhel72.stable.backport.3.x86_64.debug #1 Tainted: G W ------------
[ +0.000001] ---------------------------------------------
[ +0.000001] kworker/4:2/1262 is trying to acquire lock:
[ +0.000001] ((&mgr->work)){+.+.+.}, at: [<ffffffff810b29a5>] flush_work+0x5/0x2e0
[ +0.000007]
but task is already holding lock:
[ +0.000001] ((&mgr->work)){+.+.+.}, at: [<ffffffff810b57e4>] process_one_work+0x1b4/0x710
[ +0.000004]
other info that might help us debug this:
[ +0.000001] Possible unsafe locking scenario:

[ +0.000002] CPU0
[ +0.000000] ----
[ +0.000001] lock((&mgr->work));
[ +0.000002] lock((&mgr->work));
[ +0.000001]
*** DEADLOCK ***

[ +0.000001] May be due to missing lock nesting notation

[ +0.000002] 2 locks held by kworker/4:2/1262:
[ +0.000001] #0: (events_long){.+.+.+}, at: [<ffffffff810b57e4>] process_one_work+0x1b4/0x710
[ +0.000004] #1: ((&mgr->work)){+.+.+.}, at: [<ffffffff810b57e4>] process_one_work+0x1b4/0x710
[ +0.000003]
stack backtrace:
[ +0.000003] CPU: 4 PID: 1262 Comm: kworker/4:2 Tainted: G W ------------ 3.10.0-320.el7.rhel72.stable.backport.3.x86_64.debug #1
[ +0.000001] Hardware name: LENOVO 20EGS0R600/20EGS0R600, BIOS GNET71WW (2.19 ) 02/05/2015
[ +0.000008] Workqueue: events_long drm_dp_mst_link_probe_work [drm_kms_helper]
[ +0.000001] ffffffff82c26c90 00000000a527b914 ffff88046399bae8 ffffffff816fe04d
[ +0.000004] ffff88046399bb58 ffffffff8110f47f ffff880461438000 0001009b840fc003
[ +0.000002] ffff880461438a98 0000000000000000 0000000804dc26e1 ffffffff824a2c00
[ +0.000003] Call Trace:
[ +0.000004] [<ffffffff816fe04d>] dump_stack+0x19/0x1b
[ +0.000004] [<ffffffff8110f47f>] __lock_acquire+0x115f/0x1250
[ +0.000002] [<ffffffff8110fd49>] lock_acquire+0x99/0x1e0
[ +0.000002] [<ffffffff810b29a5>] ? flush_work+0x5/0x2e0
[ +0.000002] [<ffffffff810b29ee>] flush_work+0x4e/0x2e0
[ +0.000002] [<ffffffff810b29a5>] ? flush_work+0x5/0x2e0
[ +0.000004] [<ffffffff81025905>] ? native_sched_clock+0x35/0x80
[ +0.000002] [<ffffffff81025959>] ? sched_clock+0x9/0x10
[ +0.000002] [<ffffffff810da1f5>] ? local_clock+0x25/0x30
[ +0.000002] [<ffffffff8110dca9>] ? mark_held_locks+0xb9/0x140
[ +0.000003] [<ffffffff810b4ed5>] ? __cancel_work_timer+0x95/0x160
[ +0.000002] [<ffffffff810b4ee8>] __cancel_work_timer+0xa8/0x160
[ +0.000002] [<ffffffff810b4fb0>] cancel_work_sync+0x10/0x20
[ +0.000007] [<ffffffffa0160d17>] drm_dp_destroy_mst_branch_device+0x27/0x120 [drm_kms_helper]
[ +0.000006] [<ffffffffa0163968>] drm_dp_mst_link_probe_work+0x78/0xa0 [drm_kms_helper]
[ +0.000002] [<ffffffff810b5850>] process_one_work+0x220/0x710
[ +0.000002] [<ffffffff810b57e4>] ? process_one_work+0x1b4/0x710
[ +0.000005] [<ffffffff810b5e5b>] worker_thread+0x11b/0x3a0
[ +0.000003] [<ffffffff810b5d40>] ? process_one_work+0x710/0x710
[ +0.000002] [<ffffffff810beced>] kthread+0xed/0x100
[ +0.000003] [<ffffffff810bec00>] ? insert_kthread_work+0x80/0x80
[ +0.000003] [<ffffffff817121d8>] ret_from_fork+0x58/0x90

v2: add flush_work.

Reviewed-by: Daniel Vetter <[e-mail]>
Cc: [e-mail]
Signed-off-by: Dave Airlie <[e-mail]>

drivers/gpu/drm/drm_dp_mst_topology.c

···
 	struct drm_dp_mst_port *port, *tmp;
 	bool wake_tx = false;
 
-	cancel_work_sync(&mstb->mgr->work);
-
 	/*
 	 * destroy all ports - don't need lock
 	 * as there are no more references to the mst branch
···
 	drm_dp_dpcd_writeb(mgr->aux, DP_MSTM_CTRL,
 			   DP_MST_EN | DP_UPSTREAM_IS_SRC);
 	mutex_unlock(&mgr->lock);
+	flush_work(&mgr->work);
+	flush_work(&mgr->destroy_connector_work);
 }
 EXPORT_SYMBOL(drm_dp_mst_topology_mgr_suspend);
 
···
  */
 void drm_dp_mst_topology_mgr_destroy(struct drm_dp_mst_topology_mgr *mgr)
 {
+	flush_work(&mgr->work);
 	flush_work(&mgr->destroy_connector_work);
 	mutex_lock(&mgr->payload_lock);
 	kfree(mgr->payloads);