Re: [PATCH] f2fs: quota: fix potential deadlock

[Chao Yu <chao@kernel.org>]

On 2022/1/4 21:18, Greg KH wrote:
> On Tue, Jan 04, 2022 at 09:05:13PM +0800, Chao Yu wrote:
>> commit a5c0042200b28fff3bde6fa128ddeaef97990f8d upstream.
>>
>> As Yi Zhuang reported in bugzilla:
>>
>> https://bugzilla.kernel.org/show_bug.cgi?id=214299
>>
>> There is potential deadlock during quota data flush as below:
>>
>> Thread A:			Thread B:
>> f2fs_dquot_acquire
>> down_read(&sbi->quota_sem)
>> 				f2fs_write_checkpoint
>> 				block_operations
>> 				f2fs_look_all
>> 				down_write(&sbi->cp_rwsem)
>> f2fs_quota_write
>> f2fs_write_begin
>> __do_map_lock
>> f2fs_lock_op
>> down_read(&sbi->cp_rwsem)
>> 				__need_flush_qutoa
>> 				down_write(&sbi->quota_sem)
>>
>> This patch changes block_operations() to use trylock, if it fails,
>> it means there is potential quota data updater, in this condition,
>> let's flush quota data first and then trylock again to check dirty
>> status of quota data.
>>
>> The side effect is: in heavy race condition (e.g. multi quota data
>> upaters vs quota data flusher), it may decrease the probability of
>> synchronizing quota data successfully in checkpoint() due to limited
>> retry time of quota flush.
>>
>> Fixes: db6ec53b7e03 ("f2fs: add a rw_sem to cover quota flag changes")
>> Cc: stable@vger.kernel.org # v5.3+
>> Reported-by: Yi Zhuang <zhuangyi1@huawei.com>
>> Signed-off-by: Chao Yu <chao@kernel.org>
>> Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
>> ---
>>   fs/f2fs/checkpoint.c | 3 ++-
>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/fs/f2fs/checkpoint.c b/fs/f2fs/checkpoint.c
>> index 83e9bc0f91ff..7b0282724231 100644
>> --- a/fs/f2fs/checkpoint.c
>> +++ b/fs/f2fs/checkpoint.c
>> @@ -1162,7 +1162,8 @@ static bool __need_flush_quota(struct f2fs_sb_info *sbi)
>>   	if (!is_journalled_quota(sbi))
>>   		return false;
>>   
>> -	down_write(&sbi->quota_sem);
>> +	if (!down_write_trylock(&sbi->quota_sem))
>> +		return true;
>>   	if (is_sbi_flag_set(sbi, SBI_QUOTA_SKIP_FLUSH)) {
>>   		ret = false;
>>   	} else if (is_sbi_flag_set(sbi, SBI_QUOTA_NEED_REPAIR)) {
>> -- 
>> 2.32.0
>>
> 
> What stable tree(s) is this for?

Oh, please help to try applying to 5.4, 5.10, and 5.15 stable trees, thanks!

Let me know if I should send patches for different trees separately.

Thanks,

> 
> thanks,
> 
> greg k-h