Re: [PATCH-block] blk-cgroup: Use css_tryget() in blkcg_destroy_blkgs()

[Jens Axboe <axboe@kernel.dk>]

On 11/27/22 8:30 PM, Waiman Long wrote:
> Commit 951d1e94801f ("blk-cgroup: Flush stats at blkgs destruction
> path") incorrectly assumes that css_get() will always succeed. That may
> not be true if there is no blkg associated with the blkcg. If css_get()
> fails, the subsequent css_put() call may lead to data corruption as
> was illustrated in a test system that it crashed on bootup when that
> commit was included. Also blkcg may be freed at any time leading to
> use-after-free. Fix it by using css_tryget() instead and bail out if
> the tryget fails.
> 
> Fixes: 951d1e94801f ("blk-cgroup: Flush stats at blkgs destruction path")
> Reported-by: Yi Zhang <yi.zhang@redhat.com>
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>  block/blk-cgroup.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
> index 57941d2a8ba3..74fefc8cbcdf 100644
> --- a/block/blk-cgroup.c
> +++ b/block/blk-cgroup.c
> @@ -1088,7 +1088,12 @@ static void blkcg_destroy_blkgs(struct blkcg *blkcg)
>  
>  	might_sleep();
>  
> -	css_get(&blkcg->css);
> +	/*
> +	 * If css_tryget() fails, there is no blkg to destroy.
> +	 */
> +	if (!css_tryget(&blkcg->css))
> +		return;
> +
>  	spin_lock_irq(&blkcg->lock);
>  	while (!hlist_empty(&blkcg->blkg_list)) {
>  		struct blkcg_gq *blkg = hlist_entry(blkcg->blkg_list.first,

This doesn't seem safe to me, but maybe I'm missing something. A tryget
operation can be fine if we're under RCU lock and the entity is freed
appropriately, but what makes it safe here? Could blkcg already be gone
at this point?

-- 
Jens Axboe