From: Jiri Kosina <jkosina@suse.cz>
Subject: [PATCH] block/floppy: fix contended case in floppy_queue_rq()
Patch-mainline: v5.8-rc1
Git-commit: 263c61581a38d0a5ad1f5f4a9143b27d68caeffd
References: bsc#1171426

Since the switch of floppy driver to blk-mq, the contended (fdc_busy)
case in floppy_queue_rq() is not handled correctly.

In case we reach floppy_queue_rq() with fdc_busy set (i.e. with the floppy
locked due to another request still being in-flight), we put the request on
the list of requests and return BLK_STS_OK to the block core, without actually
scheduling delayed work / doing further processing of the request. This means
that processing of this request is postponed until another request comes and
passess uncontended.

Which in some cases might actually never happen and we keep waiting
indefinitely. The simple testcase is

	for i in `seq 1 2000`; do echo -en $i '\r'; blkid --info /dev/fd0 2> /dev/null; done

run in quemu. That reliably causes blkid eventually indefinitely hanging in
__floppy_read_block_0() waiting for completion, as the BIO callback never
happens, and no further IO is ever submitted on the (non-existent) floppy
device. This was observed reliably on qemu-emulated device.

Fix that by not queuing the request in the contended case, and return
BLK_STS_RESOURCE instead, so that blk core handles the request rescheduling
and let it pass properly non-contended later.

Fixes: a9f38e1dec107a ("floppy: convert to blk-mq")
Cc: stable@vger.kernel.org
Tested-by: Libor Pechacek <lpechacek@suse.cz>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 drivers/block/floppy.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/drivers/block/floppy.c b/drivers/block/floppy.c
index c3daa64cb52c..975cd0a6baa1 100644
--- a/drivers/block/floppy.c
+++ b/drivers/block/floppy.c
@@ -2938,17 +2938,17 @@ static blk_status_t floppy_queue_rq(struct blk_mq_hw_ctx *hctx,
 		 (unsigned long long) current_req->cmd_flags))
 		return BLK_STS_IOERR;
 
-	spin_lock_irq(&floppy_lock);
-	list_add_tail(&bd->rq->queuelist, &floppy_reqs);
-	spin_unlock_irq(&floppy_lock);
-
 	if (test_and_set_bit(0, &fdc_busy)) {
 		/* fdc busy, this new request will be treated when the
 		   current one is done */
 		is_alive(__func__, "old request running");
-		return BLK_STS_OK;
+		return BLK_STS_RESOURCE;
 	}
 
+	spin_lock_irq(&floppy_lock);
+	list_add_tail(&bd->rq->queuelist, &floppy_reqs);
+	spin_unlock_irq(&floppy_lock);
+
 	command_status = FD_COMMAND_NONE;
 	__reschedule_timeout(MAXTIMEOUT, "fd_request");
 	set_fdc(0);
-- 
2.12.3