请教关于 sharedLock 和 lockForUpdate 的区别

version:laravel 5.6
路径:Illuminate\Database\Query\Builder

 /**
     * Lock the selected rows in the table.
     *
     * @param  string|bool  $value
     * @return $this
     */
    public function lock($value = true)
    {
        $this->lock = $value;

        if (! is_null($this->lock)) {
            $this->useWritePdo();
        }

        return $this;
    }

    /**
     * Lock the selected rows in the table for updating.
     *
     * @return \Illuminate\Database\Query\Builder
     */
    public function lockForUpdate()
    {
        return $this->lock(true);
    }

    /**
     * Share lock the selected rows in the table.
     *
     * @return \Illuminate\Database\Query\Builder
     */
    public function sharedLock()
    {
        return $this->lock(false);
    }

请问lockForUpdate 和 sharedLock的区别?
function lock里使用了is_null($this->lock)做判断的,都会运行$this->useWritePdo()
在laravel 5.3(含)前使用的是

 public function lock($value = true)
    {
        $this->lock = $value;

        if ($this->lock) {
            $this->useWritePdo();
        }

        return $this;
    }

由于这个Issue$this->useWritePdo()的if判断条件被修改了.
感谢回复