Having beaten York City 2-0 three days ago, Hull headed into the clash with momentum as they build up to the new Premier League season and a maiden voyage into European football.
The visitors looked to be on their way to victory when Long gave Hull the lead early in the second half, only for midfielder Conor Hourihane to level for Barnsley.
Both sides traded chances in an even first half, Hull's Steve Harper denying winger Dale Jennings from 25 yards inside two minutes, while at the other end Sone Aluko's long-range effort drew a strong save from Adam Davies.
The deadlock was eventually broken two minutes after the interval when Long struck the opener, charging down Davies' attempted clearance following Lewin Nyatanga's misplaced pass back to his goalkeeper.
However, Hull's advantage lasted just 10 minutes, Hourihane pouncing on a mistake from Eldin Jakupovic after Hull's replacement keeper misjudged the flight of a long ball.
Jakupovic atoned for that error by keeping out a fierce free-kick from close-season Oakwell arrival Sam Winnall, and that proved to be the last meaningful chance of the game as it finished honours even.
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