[PATCH V2 3/3] Revert "block: Fix a lockdep complaint triggered by request queue flushing"

[Ming Lei]

>From a519f421957a1205918e9bcc15087d15234e4e9f Mon Sep 17 00:00:00 2001
From: Ming Lei <ming.lei at redhat.com>
Date: Thu, 12 Nov 2020 09:56:02 +0800
Subject: [PATCH V2 3/3] Revert "block: Fix a lockdep complaint triggered by
 request queue flushing"

This reverts commit b3c6a59975415bde29cfd76ff1ab008edbf614a9.

Now we can avoid nvme-loop lockdep warning of 'lockdep possible recursive
locking' by nvme-loop's lock class, no need to apply dynamically
allocated lock class key, so revert commit b3c6a5997541("block: Fix a
lockdep complaint triggered by request queue flushing").

This way fixes horrible SCSI probe delay issue on megaraid_sas(host_tagset is 1),
and it is reported the whole probe may take more than half an hour. The reason
is that synchronize_rcu() is implied in lockdep_unregister_key() which is
called from each hctx's release handler, and some SCSI hosts can support
too many hw queues, meantime generic SCSI probe may synchronously create
and destroy lots of MQ request_queues for non-existent devices.

Another benefit is that lockdep doesn't maintain so many runtime lock class for
fq->mq_flush_lock which is per-hctx, then lock validation can be improved much.

Reported-by: Qian Cai <cai at redhat.com>
Cc: Sumit Saxena <sumit.saxena at broadcom.com>
Cc: John Garry <john.garry at huawei.com>
Cc: Kashyap Desai <kashyap.desai at broadcom.com>
Cc: Bart Van Assche <bvanassche at acm.org>
Cc: Hannes Reinecke <hare at suse.de>
Signed-off-by: Ming Lei <ming.lei at redhat.com>
---
V2:
	- add more commit log

 block/blk-flush.c | 5 -----
 block/blk.h       | 1 -
 2 files changed, 6 deletions(-)

diff --git a/block/blk-flush.c b/block/blk-flush.c
index 657743524e15..c64f049226f6 100644
--- a/block/blk-flush.c
+++ b/block/blk-flush.c
@@ -69,7 +69,6 @@
 #include <linux/blkdev.h>
 #include <linux/gfp.h>
 #include <linux/blk-mq.h>
-#include <linux/lockdep.h>
 
 #include "blk.h"
 #include "blk-mq.h"
@@ -469,9 +468,6 @@ struct blk_flush_queue *blk_alloc_flush_queue(int node, int cmd_size,
 	INIT_LIST_HEAD(&fq->flush_queue[1]);
 	INIT_LIST_HEAD(&fq->flush_data_in_flight);
 
-	lockdep_register_key(&fq->key);
-	lockdep_set_class(&fq->mq_flush_lock, &fq->key);
-
 	return fq;
 
  fail_rq:
@@ -486,7 +482,6 @@ void blk_free_flush_queue(struct blk_flush_queue *fq)
 	if (!fq)
 		return;
 
-	lockdep_unregister_key(&fq->key);
 	kfree(fq->flush_rq);
 	kfree(fq);
 }
diff --git a/block/blk.h b/block/blk.h
index dfab98465db9..806fd6537295 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -25,7 +25,6 @@ struct blk_flush_queue {
 	struct list_head	flush_data_in_flight;
 	struct request		*flush_rq;
 
-	struct lock_class_key	key;
 	spinlock_t		mq_flush_lock;
 };
 
-- 
2.25.4