linux-kernel - Re: [PATCH -next v2 2/3] blk-cgroup: support to track if policy is online

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Date:   Wed, 18 Jan 2023 06:55:03 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Yu Kuai <yukuai1@...weicloud.com>
Cc:     hch@....de, josef@...icpanda.com, axboe@...nel.dk,
        cgroups@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, yukuai3@...wei.com,
        yi.zhang@...wei.com, yangerkun@...wei.com
Subject: Re: [PATCH -next v2 2/3] blk-cgroup: support to track if policy is
 online

On Wed, Jan 18, 2023 at 08:31:51PM +0800, Yu Kuai wrote:
> From: Yu Kuai <yukuai3@...wei.com>
> 
> A new field 'online' is added to blkg_policy_date to fix following
                                                  ^
                                                  a
> 2 problem:
> 
> 1) In blkcg_activate_policy(), if pd_alloc_fn() with 'GFP_NOWAIT'
>    failed, 'queue_lock' will be dropped and pd_alloc_fn() will try again
>    without 'GFP_NOWAIT'. In the meantime, remove cgroup can race with
>    it, and pd_offline_fn() will be called without pd_init_fn() and
>    pd_online_fn(). This way null-ptr-deference can be triggered.
> 
> 2) In order to synchronize pd_free_fn() from blkg_free_workfn() and
>    blkcg_deactivate_policy(), 'list_del_init(&blkg->q_node)' will be
>    delayed to blkg_free_workfn(), hence pd_offline_fn() can be called
>    first in blkg_destroy(), and then blkcg_deactivate_policy() will
>    call it again, we must prevent it.
> 
> The new field 'online' will be set after pd_online_fn() and will be
> cleared after pd_offline_fn(), in the meantime pd_offline_fn() will only
> be called if 'online' is set.
> 
> Signed-off-by: Yu Kuai <yukuai3@...wei.com>

Acked-by: Tejun Heo <tj@...nel.org>

Thanks.

-- 
tejun