Can a prison official be held liable for failing to warn a prisoner of a substantial risk of serious harm?

drawn that a substantial risk of serious harm exists, but he must also draw the inference. Id. A prison official cannot be held liable if she was unaware of even an obvious risk, even if the harm ultimately was not averted. Farmer, 511 U.S. at 844. Moreover, conduct that does not purport to be punishment must involve more than ordinary lack of due care for the prisoner's interests or safety -- it is obduracy and wantonness, not inadvertence or error in good faith, that characterize the prohibited conduct. Whitley v. Albers, 475 U.S. 312, 319 (1986