linux-kernel - Re: [PATCH] mm: backing-dev: use kfree_rcu() instead of synchronize_rcu

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Date:   Mon, 18 Oct 2021 10:15:53 +0800
From:   Zqiang <qiang.zhang1211@...il.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     hch@...radead.org, akpm@...ux-foundation.org, sunhao.th@...il.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Mikulas Patocka <mpatocka@...hat.com>,
        Jens Axboe <axboe@...nel.dk>, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH] mm: backing-dev: use kfree_rcu() instead of
 synchronize_rcu_expedited()


On 2021/10/15 下午8:35, Matthew Wilcox wrote:
> On Fri, Oct 15, 2021 at 01:06:02PM +0800, Zqiang wrote:
>> On 2021/10/15 上午10:57, Qiang Zhang wrote:
>>>
>>> Matthew Wilcox <willy@...radead.org <mailto:willy@...radead.org>>
>>> 于2021年10月14日周四 下午7:26写道：
>>>
>>>      On Thu, Oct 14, 2021 at 04:24:33PM +0800, Zqiang wrote:
>>>      > The bdi_remove_from_list() is called in RCU softirq, however the
>>>      > synchronize_rcu_expedited() will produce sleep action, use
>>>      kfree_rcu()
>>>      > instead of it.
>>>      >
>>>      > Reported-by: Hao Sun <sunhao.th@...il.com
>>>      <mailto:sunhao.th@...il.com>>
>>>      > Signed-off-by: Zqiang <qiang.zhang1211@...il.com
>>>      <mailto:qiang.zhang1211@...il.com>>
>>>      > ---
>>>      >  include/linux/backing-dev-defs.h | 1 +
>>>      >  mm/backing-dev.c                 | 4 +---
>>>      >  2 files changed, 2 insertions(+), 3 deletions(-)
>>>      >
>>>      > diff --git a/include/linux/backing-dev-defs.h
>>>      b/include/linux/backing-dev-defs.h
>>>      > index 33207004cfde..35a093384518 100644
>>>      > --- a/include/linux/backing-dev-defs.h
>>>      > +++ b/include/linux/backing-dev-defs.h
>>>      > @@ -202,6 +202,7 @@ struct backing_dev_info {
>>>      >  #ifdef CONFIG_DEBUG_FS
>>>      >       struct dentry *debug_dir;
>>>      >  #endif
>>>      > +     struct rcu_head rcu;
>>>      >  };
>>>
>>>      >Instead of growing struct backing_dev_info, it seems to me this
>>>      rcu_head
>>>      >could be placed in a union with rb_node, since it will have been
>>>      removed
>>>      >from the bdi_tree by this point and the tree is never walked under
>>>      >RCU protection?
>>>
>>>
>>> Thanks for your advice, I find this bdi_tree is traversed under the
>>> protection of a spin lock, not under the protection of RCU.
>>> I find this modification does not avoid the problem described in patch,
>>> the flush_delayed_work() may be called in release_bdi()
>>> The same will cause problems.
>>> may be  we can replace queue_rcu_work() of call_rcu(&inode->i_rcu,
>>> i_callback) or do you have any better suggestions?
> What?  All I was suggesting was:
>
> +++ b/include/linux/backing-dev-defs.h
> @@ -168,7 +168,10 @@ struct bdi_writeback {
>   
>   struct backing_dev_info {
>          u64 id;
> -       struct rb_node rb_node; /* keyed by ->id */
> +       union {
> +               struct rb_node rb_node; /* keyed by ->id */
> +               struct rcu_head rcu;
> +       };
>          struct list_head bdi_list;
>          unsigned long ra_pages; /* max readahead in PAGE_SIZE units */
>          unsigned long io_pages; /* max allowed IO size */
>
>
> Christoph, independent of the inode lifetime problem, this actually seems
> like a good approach to take.  I don't see why we should synchronize_rcu()
> here?  Adding Jens (original introducer of the synchronize_rcu()), Mikulas
> (converted it to use _expedited) and Tejun (worked around a problem when
> using _expedited).

Sorry，this my mistake.   this problem and the inode lifetime cycle are 
two different problems

Can this modification which use kfree_rcu() instead of synchronize_rcu() 
be accepted?


Thanks

Zqiang