linux-kernel - Re: [PATCH V3] f2fs: Avoid double lock for cp

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Date:   Thu, 30 Apr 2020 15:36:42 +0530
From:   Sayali Lokhande <sayalil@...eaurora.org>
To:     Chao Yu <yuchao0@...wei.com>, jaegeuk@...nel.org,
        linux-f2fs-devel@...ts.sourceforge.net
Cc:     stummala@...eaurora.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V3] f2fs: Avoid double lock for cp_rwsem during checkpoint


On 4/30/2020 6:41 AM, Chao Yu wrote:
> On 2020/4/30 1:00, Sayali Lokhande wrote:
>> There could be a scenario where f2fs_sync_node_pages gets
>> called during checkpoint, which in turn tries to flush
>> inline data and calls iput(). This results in deadlock as
>> iput() tries to hold cp_rwsem, which is already held at the
>> beginning by checkpoint->block_operations().
>>
>> Call stack :
>>
>> Thread A		Thread B
>> f2fs_write_checkpoint()
>> - block_operations(sbi)
>>   - f2fs_lock_all(sbi);
>>    - down_write(&sbi->cp_rwsem);
>>
>>                          - open()
>>                           - igrab()
>>                          - write() write inline data
>>                          - unlink()
>> - f2fs_sync_node_pages()
>>   - if (is_inline_node(page))
>>    - flush_inline_data()
>>     - ilookup()
>>       page = f2fs_pagecache_get_page()
>>       if (!page)
>>        goto iput_out;
>>       iput_out:
>> 			-close()
>> 			-iput()
>>         iput(inode);
>>         - f2fs_evict_inode()
>>          - f2fs_truncate_blocks()
>>           - f2fs_lock_op()
>>             - down_read(&sbi->cp_rwsem);
>>
>> Fixes: 399368372ed9 ("f2fs: introduce a new global lock scheme")
> IMO, it should be
>
> 2049d4fcb057 ("f2fs: avoid multiple node page writes due to inline_data")
>
> It brings iput() to checkpoint process for the first time.
>
> Thanks,
Agreed. will update it.
>> Signed-off-by: Sayali Lokhande <sayalil@...eaurora.org>
>> ---
>>   fs/f2fs/checkpoint.c | 10 ++++------
>>   1 file changed, 4 insertions(+), 6 deletions(-)
>>
>> diff --git a/fs/f2fs/checkpoint.c b/fs/f2fs/checkpoint.c
>> index 5ba649e..97b6378 100644
>> --- a/fs/f2fs/checkpoint.c
>> +++ b/fs/f2fs/checkpoint.c
>> @@ -1219,21 +1219,19 @@ static int block_operations(struct f2fs_sb_info *sbi)
>>   		goto retry_flush_quotas;
>>   	}
>>   
>> -retry_flush_nodes:
>>   	down_write(&sbi->node_write);
>>   
>>   	if (get_pages(sbi, F2FS_DIRTY_NODES)) {
>>   		up_write(&sbi->node_write);
>> +		up_write(&sbi->node_change);
>> +		f2fs_unlock_all(sbi);
>>   		atomic_inc(&sbi->wb_sync_req[NODE]);
>>   		err = f2fs_sync_node_pages(sbi, &wbc, false, FS_CP_NODE_IO);
>>   		atomic_dec(&sbi->wb_sync_req[NODE]);
>> -		if (err) {
>> -			up_write(&sbi->node_change);
>> -			f2fs_unlock_all(sbi);
>> +		if (err)
>>   			goto out;
>> -		}
>>   		cond_resched();
>> -		goto retry_flush_nodes;
>> +		goto retry_flush_quotas;
>>   	}
>>   
>>   	/*
>>