block: fix request.queuelist usage in flush

JIRA: https://issues.redhat.com/browse/RHEL-43552

commit d0321c8
Author: Chengming Zhou <chengming.zhou@linux.dev>
Date:   Sat Jun 8 22:31:15 2024 +0800

    block: fix request.queuelist usage in flush

    Friedrich Weber reported a kernel crash problem and bisected to commit
    81ada09 ("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 81ada09 ("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.

    This updated version adds "list_del_init(&rq->queuelist)" in flush rq
    callback since the dm layer may submit request of a weird invalid format
    (REQ_FSEQ_PREFLUSH | REQ_FSEQ_POSTFLUSH), which causes double list_add
    if without this "list_del_init(&rq->queuelist)". The weird invalid format
    problem should be fixed in dm layer.

    Reported-by: Friedrich Weber <f.weber@proxmox.com>
    Closes: https://lore.kernel.org/lkml/14b89dfb-505c-49f7-aebb-01c54451db40@proxmox.com/
    Closes: https://lore.kernel.org/lkml/c9d03ff7-27c5-4ebd-b3f6-5a90d96f35ba@proxmox.com/
    Fixes: 81ada09 ("blk-flush: reuse rq queuelist in flush state machine")
    Cc: Christoph Hellwig <hch@lst.de>
    Cc: ming.lei@redhat.com
    Cc: bvanassche@acm.org
    Tested-by: Friedrich Weber <f.weber@proxmox.com>
    Signed-off-by: Chengming Zhou <chengming.zhou@linux.dev>
    Reviewed-by: Christoph Hellwig <hch@lst.de>
    Link: https://lore.kernel.org/r/20240608143115.972486-1-chengming.zhou@linux.dev
    Signed-off-by: Jens Axboe <axboe@kernel.dk>

Signed-off-by: Ming Lei <ming.lei@redhat.com>

Author: Ming Lei

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:
@@ -261,6 +261,7 @@ static enum rq_end_io_ret flush_end_io(struct request *flush_rq,
 		unsigned int seq = blk_flush_cur_seq(rq);
 
 		BUG_ON(seq != REQ_FSEQ_PREFLUSH && seq != REQ_FSEQ_POSTFLUSH);
+		list_del_init(&rq->queuelist);
 		blk_flush_complete_seq(rq, fq, seq, error);
 	}