linux-kernel - Re: [PATCH] block/floppy: fix contended case in floppy_queue

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Message-ID: <nycvar.YFH.7.76.2005262031160.25812@cbobk.fhfr.pm>
Date:   Tue, 26 May 2020 20:31:50 +0200 (CEST)
From:   Jiri Kosina <jikos@...nel.org>
To:     Denis Efremov <efremov@...ux.com>, Jens Axboe <axboe@...nel.dk>,
        Omar Sandoval <osandov@...com>
cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        Libor Pechacek <lpechacek@...e.cz>
Subject: Re: [PATCH] block/floppy: fix contended case in floppy_queue_rq()


[ forgot to CC Omar, fixing ]

On Tue, 26 May 2020, Jiri Kosina wrote:

> From: Jiri Kosina <jkosina@...e.cz>
> 
> 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@...r.kernel.org
> Tested-by: Libor Pechacek <lpechacek@...e.cz>
> Signed-off-by: Jiri Kosina <jkosina@...e.cz>
> ---
>  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);
> 
> -- 
> Jiri Kosina
> SUSE Labs
> 
> 

-- 
Jiri Kosina
SUSE Labs