Re: [f2fs-dev] [PATCH] f2fs: Fix deadlock between f2fs_quota_sync and block_operation

[Chao Yu <yuchao0@huawei.com>]

On 2020/11/29 1:41, Shachar Raindel wrote:
> This deadlock is hitting Android users (Pixel 3/3a/4) with Magisk, due
> to frequent umount/mount operations that trigger quota_sync, hitting
> the race. See https://github.com/topjohnwu/Magisk/issues/3171 for
> additional impact discussion.
> 
> In commit db6ec53b7e03, we added a semaphore to protect quota flags.
> As part of this commit, we changed f2fs_quota_sync to call
> f2fs_lock_op, in an attempt to prevent an AB/BA type deadlock with
> quota_sem locking in block_operation.  However, rwsem in Linux is not
> recursive. Therefore, the following deadlock can occur:
> 
> f2fs_quota_sync
> down_read(cp_rwsem) // f2fs_lock_op
> filemap_fdatawrite
> f2fs_write_data_pages
> ...
>                                     block_opertaion
> 				   down_write(cp_rwsem) - marks rwsem as
> 				                          "writer pending"
> down_read_trylock(cp_rwsem) - fails as there is
>                                a writer pending.
> 			      Code keeps on trying,
> 			      live-locking the filesystem.

f2fs_write_single_data_page() will not grab read lock of cp_rwsem now, could you
please check f2fs code in mainline?

	/* Dentry/quota blocks are controlled by checkpoint */
	if (S_ISDIR(inode->i_mode) || IS_NOQUOTA(inode)) {
		/*
		 * We need to wait for node_write to avoid block allocation during
		 * checkpoint. This can only happen to quota writes which can cause
		 * the below discard race condition.
		 */
		if (IS_NOQUOTA(inode))
			down_read(&sbi->node_write);

		fio.need_lock = LOCK_DONE;
		err = f2fs_do_write_data_page(&fio);

		if (IS_NOQUOTA(inode))
			up_read(&sbi->node_write);

		goto done;
	}

Thanks,

> 
> We solve this by creating a new rwsem, used specifically to
> synchronize this case, instead of attempting to reuse an existing
> lock.
> 
> Signed-off-by: Shachar Raindel <shacharr@gmail.com>
> 
> Fixes: db6ec53b7e03 f2fs: add a rw_sem to cover quota flag changes
> ---
>   fs/f2fs/f2fs.h  |  3 +++
>   fs/f2fs/super.c | 13 +++++++++++--
>   2 files changed, 14 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
> index cb700d797296..b3e55137be7f 100644
> --- a/fs/f2fs/f2fs.h
> +++ b/fs/f2fs/f2fs.h
> @@ -1448,6 +1448,7 @@ struct f2fs_sb_info {
>   	struct inode *meta_inode;		/* cache meta blocks */
>   	struct mutex cp_mutex;			/* checkpoint procedure lock */
>   	struct rw_semaphore cp_rwsem;		/* blocking FS operations */
> +	struct rw_semaphore cp_quota_rwsem;    	/* blocking quota sync operations */
>   	struct rw_semaphore node_write;		/* locking node writes */
>   	struct rw_semaphore node_change;	/* locking node change */
>   	wait_queue_head_t cp_wait;
> @@ -1961,12 +1962,14 @@ static inline void f2fs_unlock_op(struct f2fs_sb_info *sbi)
>   
>   static inline void f2fs_lock_all(struct f2fs_sb_info *sbi)
>   {
> +	down_write(&sbi->cp_quota_rwsem);
>   	down_write(&sbi->cp_rwsem);
>   }
>   
>   static inline void f2fs_unlock_all(struct f2fs_sb_info *sbi)
>   {
>   	up_write(&sbi->cp_rwsem);
> +	up_write(&sbi->cp_quota_rwsem);
>   }
>   
>   static inline int __get_cp_reason(struct f2fs_sb_info *sbi)
> diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
> index 00eff2f51807..5ce61147d7e5 100644
> --- a/fs/f2fs/super.c
> +++ b/fs/f2fs/super.c
> @@ -2209,8 +2209,16 @@ int f2fs_quota_sync(struct super_block *sb, int type)
>   	 *  f2fs_dquot_commit
>   	 *                            block_operation
>   	 *                            down_read(quota_sem)
> +	 *
> +	 * However, we cannot use the cp_rwsem to prevent this
> +	 * deadlock, as the cp_rwsem is taken for read inside the
> +	 * f2fs_dquot_commit code, and rwsem is not recursive.
> +	 *
> +	 * We therefore use a special lock to synchronize
> +	 * f2fs_quota_sync with block_operations, as this is the only
> +	 * place where such recursion occurs.
>   	 */
> -	f2fs_lock_op(sbi);
> +	down_read(&sbi->cp_quota_rwsem);
>   
>   	down_read(&sbi->quota_sem);
>   	ret = dquot_writeback_dquots(sb, type);
> @@ -2251,7 +2259,7 @@ int f2fs_quota_sync(struct super_block *sb, int type)
>   	if (ret)
>   		set_sbi_flag(F2FS_SB(sb), SBI_QUOTA_NEED_REPAIR);
>   	up_read(&sbi->quota_sem);
> -	f2fs_unlock_op(sbi);
> +	up_read(&sbi->cp_quota_rwsem);
>   	return ret;
>   }
>   
> @@ -3599,6 +3607,7 @@ static int f2fs_fill_super(struct super_block *sb, void *data, int silent)
>   
>   	init_rwsem(&sbi->cp_rwsem);
>   	init_rwsem(&sbi->quota_sem);
> +	init_rwsem(&sbi->cp_quota_rwsem);
>   	init_waitqueue_head(&sbi->cp_wait);
>   	init_sb_info(sbi);
>   
>