Re: [PATCH] rnbd: fix compilation error when CONFIG_MODULES is disabled

[Jason Gunthorpe <jgg@ziepe.ca>]

On Thu, May 21, 2020 at 08:59:09PM +0200, Danil Kipnis wrote:
> module_is_live function is only defined when CONFIG_MODULES is enabled.
> Use try_module_get instead to check whether the module is being removed.
> 
> When module unload and manuall unmapping is happening in parallel, we can
> try removing the symlink twice: rnbd_client_exit vs. rnbd_clt_unmap_dev_store.
> 
> This is probably not the best way to deal with this race in general, but for
> now this fixes the compilation issue when CONFIG_MODULES is disabled and has
> no functional impact. Regression tests passed.
> 
> Fixes: 1eb54f8f5dd8 ("block/rnbd: client: sysfs interface functions")
> Reported-by: Randy Dunlap <rdunlap@infradead.org>
> Suggested-by: Guoqing Jiang <guoqing.jiang@cloud.ionos.com>
> Signed-off-by: Danil Kipnis <danil.kipnis@cloud.ionos.com>
> Acked-by: Randy Dunlap <rdunlap@infradead.org>
> v1->v2 Fix format of the "Fixes:" line
>        Add Acked-by Randy Runlap <rdunlap@infradead.org>
>  drivers/block/rnbd/rnbd-clt-sysfs.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/block/rnbd/rnbd-clt-sysfs.c b/drivers/block/rnbd/rnbd-clt-sysfs.c
> index a4508fcc7ffe..73d7cb40abb3 100644
> +++ b/drivers/block/rnbd/rnbd-clt-sysfs.c
> @@ -428,12 +428,14 @@ static struct attribute *rnbd_dev_attrs[] = {
>  void rnbd_clt_remove_dev_symlink(struct rnbd_clt_dev *dev)
>  {
>  	/*
> -	 * The module_is_live() check is crucial and helps to avoid annoying
> -	 * sysfs warning raised in sysfs_remove_link(), when the whole sysfs
> -	 * path was just removed, see rnbd_close_sessions().
> +	 * The module unload rnbd_client_exit path is racing with unmapping of the
> +	 * last single device from the sysfs manually i.e. rnbd_clt_unmap_dev_store()
> +	 * leading to a sysfs warning because of sysfs link already was removed already.
>  	 */
> -	if (strlen(dev->blk_symlink_name) && module_is_live(THIS_MODULE))
> +	if (strlen(dev->blk_symlink_name) && try_module_get(THIS_MODULE)) {
>  		sysfs_remove_link(rnbd_devs_kobj, dev->blk_symlink_name);
> +		module_put(THIS_MODULE);
> +	}
>  }

This is a really gross thing to do, please fix it properly in future

Applied to for-next

Jason