There is a tricky interaction between the machine check handler and the critical sections of load_fpu_regs and save_fpu_regs functions. If the machine check interrupts one of the two functions the critical section cleanup will complete the function before the machine check handler s390_do_machine_check is called. Trouble is that the machine check handler needs to validate the floating point registers *before* and not *after* the completion of load_fpu_regs/save_fpu_regs. The simplest solution is to rewind the PSW to the start of the load_fpu_regs/save_fpu_regs and retry the function after the return from the machine check handler. Tested-by: Christian Borntraeger <borntraeger@de.ibm.com> Cc: <stable@vger.kernel.org> # 4.3+ Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>