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

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Date:   Fri, 15 Oct 2021 11:39:40 +0800
From:   zhangqiang <zhangqiang@...aft.ai>
To:     Matthew Wilcox <willy@...radead.org>,
        Zqiang <qiang.zhang1211@...il.com>, hch@...radead.org
Cc:     akpm@...ux-foundation.org, sunhao.th@...il.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] mm: backing-dev: use kfree_rcu() instead of
 synchronize_rcu_expedited()


On 2021/10/14 下午7:24, Matthew Wilcox wrote:
> 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>
>> Signed-off-by: Zqiang <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?

Thanks
Zqiang