Re: [PATCH] sd: Fix a race between closing an sd device and sd I/O

[Ming Lei <ming.lei@redhat.com>]

On Mon, Mar 25, 2019 at 10:01:46AM -0700, Bart Van Assche wrote:
> The scsi_end_request() function calls scsi_cmd_to_driver() indirectly
> and hence needs the disk->private_data pointer. Avoid that that pointer
> is cleared before all affected I/O requests have finished. This patch
> avoids that the following crash occurs:
> 
> Unable to handle kernel NULL pointer dereference at virtual address 0000000000000000
> Call trace:
>  scsi_mq_uninit_cmd+0x1c/0x30
>  scsi_end_request+0x7c/0x1b8
>  scsi_io_completion+0x464/0x668
>  scsi_finish_command+0xbc/0x160
>  scsi_eh_flush_done_q+0x10c/0x170
>  sas_scsi_recover_host+0x84c/0xa98 [libsas]
>  scsi_error_handler+0x140/0x5b0
>  kthread+0x100/0x12c
>  ret_from_fork+0x10/0x18
> 
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Ming Lei <ming.lei@redhat.com>
> Cc: Hannes Reinecke <hare@suse.com>
> Cc: Johannes Thumshirn <jthumshirn@suse.de>
> Cc: Jason Yan <yanaijie@huawei.com>
> Cc: <stable@vger.kernel.org>
> Reported-by: Jason Yan <yanaijie@huawei.com>
> Signed-off-by: Bart Van Assche <bvanassche@acm.org>
> ---
>  drivers/scsi/sd.c | 19 +++++++++++++------
>  1 file changed, 13 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/scsi/sd.c b/drivers/scsi/sd.c
> index ed34bfbc3844..0077880c0cc8 100644
> --- a/drivers/scsi/sd.c
> +++ b/drivers/scsi/sd.c
> @@ -1416,11 +1416,6 @@ static void sd_release(struct gendisk *disk, fmode_t mode)
>  			scsi_set_medium_removal(sdev, SCSI_REMOVAL_ALLOW);
>  	}
>  
> -	/*
> -	 * XXX and what if there are packets in flight and this close()
> -	 * XXX is followed by a "rmmod sd_mod"?
> -	 */
> -
>  	scsi_disk_put(sdkp);
>  }
>  
> @@ -3483,9 +3478,21 @@ static void scsi_disk_release(struct device *dev)
>  {
>  	struct scsi_disk *sdkp = to_scsi_disk(dev);
>  	struct gendisk *disk = sdkp->disk;
> -	
> +	struct request_queue *q = disk->queue;
> +
>  	ida_free(&sd_index_ida, sdkp->index);
>  
> +	/*
> +	 * Wait until all requests that are in progress have completed.
> +	 * This is necessary to avoid that e.g. scsi_end_request() crashes
> +	 * due to clearing the disk->private_data pointer. Wait from inside
> +	 * scsi_disk_release() instead of from sd_release() to avoid that
> +	 * freezing and unfreezing the request queue affects user space I/O
> +	 * in case multiple processes open a /dev/sd... node concurrently.
> +	 */
> +	blk_mq_freeze_queue(q);
> +	blk_mq_unfreeze_queue(q);
> +
>  	disk->private_data = NULL;
>  	put_disk(disk);
>  	put_device(&sdkp->device->sdev_gendev);

No, this way may cause big performance issue, see my previous comment:

https://marc.info/?l=linux-scsi&m=155321977714715&w=2


Thanks,
Ming