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

[Greg KH <gregkh@linuxfoundation.org>]

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?

thanks,

greg k-h