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SubjectRe: [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@suse.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@vger.kernel.org
> Tested-by: Libor Pechacek <lpechacek@suse.cz>
> Signed-off-by: Jiri Kosina <jkosina@suse.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

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