backport fix for managing block flush queue list

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

patches/kernel/0016-block-fix-request.queuelist-usage-in-flush.patch

@@ -0,0 +1,48 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Chengming Zhou <chengming.zhou@linux.dev>
+Date: Tue, 4 Jun 2024 14:47:45 +0800
+Subject: [PATCH] block: fix request.queuelist usage in flush
+
+Friedrich Weber reported a kernel crash problem and bisected to commit
+81ada09cc25e ("blk-flush: reuse rq queuelist in flush state machine").
+
+The root cause is that we use "list_move_tail(&rq->queuelist, pending)"
+in the PREFLUSH/POSTFLUSH sequences. But rq->queuelist.next == xxx since
+it's popped out from plug->cached_rq in __blk_mq_alloc_requests_batch().
+We don't initialize its queuelist just for this first request, although
+the queuelist of all later popped requests will be initialized.
+
+Fix it by changing to use "list_add_tail(&rq->queuelist, pending)" so
+rq->queuelist doesn't need to be initialized. It should be ok since rq
+can't be on any list when PREFLUSH or POSTFLUSH, has no move actually.
+
+Please note the commit 81ada09cc25e ("blk-flush: reuse rq queuelist in
+flush state machine") also has another requirement that no drivers would
+touch rq->queuelist after blk_mq_end_request() since we will reuse it to
+add rq to the post-flush pending list in POSTFLUSH. If this is not true,
+we will have to revert that commit IMHO.
+
+Reported-by: Friedrich Weber <f.weber@proxmox.com>
+Closes: https://lore.kernel.org/lkml/14b89dfb-505c-49f7-aebb-01c54451db40@proxmox.com/
+Fixes: 81ada09cc25e ("blk-flush: reuse rq queuelist in flush state machine")
+Cc: Christoph Hellwig <hch@lst.de>
+Cc: ming.lei@redhat.com
+Cc: bvanassche@acm.org
+Signed-off-by: Chengming Zhou <chengming.zhou@linux.dev>
+---
+ block/blk-flush.c | 2 +-
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+
+diff --git a/block/blk-flush.c b/block/blk-flush.c
+index 3f4d41952ef2..aa5e573dd010 100644
+--- a/block/blk-flush.c
++++ b/block/blk-flush.c
+@@ -183,7 +183,7 @@ static void blk_flush_complete_seq(struct request *rq,
+ 		/* queue for flush */
+ 		if (list_empty(pending))
+ 			fq->flush_pending_since = jiffies;
+-		list_move_tail(&rq->queuelist, pending);
++		list_add_tail(&rq->queuelist, pending);
+ 		break;
+ 
+ 	case REQ_FSEQ_DATA: