Re: [RFC PATCH] SCSI: fix queue cleanup race before queue is initialized done

jianchao.wang
2018-11-14T05:41:56Z

On 11/14/18 12:35 PM, Ming Lei wrote:
> c2856ae2f315d ("blk-mq: quiesce queue before freeing queue") has
> already fixed this race, however the implied synchronize_rcu()
> in blk_mq_quiesce_queue() can slow down LUN probe a lot, so caused
> performance regression.
>
> Then 1311326cf4755c7 ("blk-mq: avoid to synchronize rcu inside blk_cleanup_queue()")
> tried to only quiesce queue for avoiding unnecessary synchronize_rcu()
> in case that queue isn't initialized done.
>
> However, turns out we still need to quiesce the queue in case that
> queue isn't initialized done. Because when one SCSI command is
> completed, the user is waken up immediately, then the scsi device
> can be removed, meantime the run queue in scsi_end_request() can
> be still in-progress, so kernel panic is triggered.
>
> In Red Hat QE lab, there are several reports about this kind of kernel
> panic triggered during kernel booting.
>
> Fixes: 1311326cf4755c7 ("blk-mq: avoid to synchronize rcu inside blk_cleanup_queue()")
> Cc: Andrew Jones <drjones@redhat.com>
> Cc: Bart Van Assche <bart.vanassche@wdc.com>
> Cc: linux-scsi@vger.kernel.org
> Cc: Martin K. Petersen <martin.petersen@oracle.com>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: James E.J. Bottomley <jejb@linux.vnet.ibm.com>
> Cc: stable <stable@vger.kernel.org>
> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> ---
> block/blk-core.c | 6 +++---
> drivers/scsi/scsi_lib.c | 36 ++++++++++++++++++++++++++++++------
> 2 files changed, 33 insertions(+), 9 deletions(-)
>
> diff --git a/block/blk-core.c b/block/blk-core.c
> index ce12515f9b9b..cf7742a677c4 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -798,9 +798,9 @@ void blk_cleanup_queue(struct request_queue *q)
> * dispatch may still be in-progress since we dispatch requests
> * from more than one contexts.
> *
> - * No need to quiesce queue if it isn't initialized yet since
> - * blk_freeze_queue() should be enough for cases of passthrough
> - * request.
> + * We rely on driver to deal with the race in case that queue
> + * initialization isn't done.
> + *
> */
> if (q->mq_ops && blk_queue_init_done(q))
> blk_mq_quiesce_queue(q);
> diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
> index c7fccbb8f554..7ec7a8a2d000 100644
> --- a/drivers/scsi/scsi_lib.c
> +++ b/drivers/scsi/scsi_lib.c
> @@ -697,13 +697,37 @@ static bool scsi_end_request(struct request *req, blk_status_t error,
> */
> scsi_mq_uninit_cmd(cmd);
>
> - __blk_mq_end_request(req, error);
> + /*
> + * When block queue initialization isn't done, the request
> + * queue won't be quiesced in blk_cleanup_queue() for avoiding
> + * slowing down LUN probe, so queue still may be run even though
> + * its resource is cleaned up, this way can cause kernel panic.
> + *
> + * Workaround this issue by freeing request after running the
> + * queue when queue initialization isn't done, so the queue's
> + * usage counter can be held during running queue.
> + *
> + * This way is safe because sdev->device_busy has been decreased
> + * already, and scsi_queue_rq() may guarantee the forward-progress.
> + *
> + */
> + if (blk_queue_init_done(q)) {
> + __blk_mq_end_request(req, error);
> +
> + if (scsi_target(sdev)->single_lun ||
> + !list_empty(&sdev->host->starved_list))
> + kblockd_schedule_work(&sdev->requeue_work);
> + else
> + blk_mq_run_hw_queues(q, true);
> + } else {
>
> - if (scsi_target(sdev)->single_lun ||
> - !list_empty(&sdev->host->starved_list))
> - kblockd_schedule_work(&sdev->requeue_work);
> - else
> - blk_mq_run_hw_queues(q, true);
> + if (scsi_target(sdev)->single_lun ||
> + !list_empty(&sdev->host->starved_list))
> + kblockd_schedule_work(&sdev->requeue_work);
> + else
> + blk_mq_run_hw_queues(q, true);
> + __blk_mq_end_request(req, error);
> + }
> } else {
> unsigned long flags;
>
>

Why not get a q_usage_counter during this ?

Something like，

@@ -610,6 +610,7 @@ static bool scsi_end_request(struct request *req, blk_status_t error,
*/
scsi_mq_uninit_cmd(cmd);

+ percpu_ref_get(&q->q_usage_counter)
__blk_mq_end_request(req, error);

if (scsi_target(sdev)->single_lun ||
@@ -618,6 +619,8 @@ static bool scsi_end_request(struct request *req, blk_status_t error,
else
blk_mq_run_hw_queues(q, true);

+ percpu_ref_put(&q->q_usage_counter)
+
put_device(&sdev->sdev_gendev);
return false;
}

Thanks
Jianchao