linux-kernel - Re: [PATCH v3] writeback: fix obtain a reference to a freeing memcg css

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Date:   Sun, 4 Apr 2021 14:01:26 -0400
From:   Tejun Heo <tj@...nel.org>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     viro@...iv.linux.org.uk, axboe@...com, willy@...radead.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        duanxiongchun@...edance.com, Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH v3] writeback: fix obtain a reference to a freeing memcg
 css

On Fri, Apr 02, 2021 at 05:11:45PM +0800, Muchun Song wrote:
> The caller of wb_get_create() should pin the memcg, because
> wb_get_create() relies on this guarantee. The rcu read lock
> only can guarantee that the memcg css returned by css_from_id()
> cannot be released, but the reference of the memcg can be zero.
> 
>   rcu_read_lock()
>   memcg_css = css_from_id()
>   wb_get_create(memcg_css)
>       cgwb_create(memcg_css)
>           // css_get can change the ref counter from 0 back to 1
>           css_get(memcg_css)
>   rcu_read_unlock()
> 
> Fix it by holding a reference to the css before calling
> wb_get_create(). This is not a problem I encountered in the
> real world. Just the result of a code review.
> 
> Fixes: 682aa8e1a6a1 ("writeback: implement unlocked_inode_to_wb transaction and use it for stat updates")
> Signed-off-by: Muchun Song <songmuchun@...edance.com>
> Acked-by: Michal Hocko <mhocko@...e.com>

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

Thanks.

-- 
tejun