From: Linus Torvalds The NMI handler uses the paranoid_exit routine that checks the NEED_RESCHED flag, and if it is set and the return is for userspace, then interrupts are enabled, the stack is swapped to the thread's stack, and schedule is called. The problem with this is that we are still in an NMI context until an iret is executed. This means that any new NMIs are now starved until an interrupt or exception occurs and does the iret. As NMIs can not be masked and can interrupt any location, they are treated as a special case. NEED_RESCHED should not be set in an NMI handler. The interruption by the NMI should not disturb the work flow for scheduling. Any IPI sent to a processor after sending the NEED_RESCHED would have to wait for the NMI anyway, and after the IPI finishes the schedule would be called as required. There is no reason to do anything special leaving an NMI. Remove the call to paranoid_exit and do a simple return. This not only fixes the bug of starved NMIs, but it also cleans up the code. Link: http://lkml.kernel.org/r/CA+55aFzgM55hXTs4griX5e9=v_O+=ue+7Rj0PTD=M7hFYpyULQ@mail.gmail.com Acked-by: Andi Kleen Cc: Ingo Molnar Cc: Peter Zijlstra Cc: "H. Peter Anvin" Cc: Frederic Weisbecker Cc: Thomas Gleixner Cc: Paul Turner Signed-off-by: Linus Torvalds --- arch/x86/kernel/entry_64.S | 32 -------------------------------- 1 files changed, 0 insertions(+), 32 deletions(-) diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S index faf8d5e..3819ea9 100644 --- a/arch/x86/kernel/entry_64.S +++ b/arch/x86/kernel/entry_64.S @@ -1489,46 +1489,14 @@ ENTRY(nmi) movq %rsp,%rdi movq $-1,%rsi call do_nmi -#ifdef CONFIG_TRACE_IRQFLAGS - /* paranoidexit; without TRACE_IRQS_OFF */ - /* ebx: no swapgs flag */ - DISABLE_INTERRUPTS(CLBR_NONE) testl %ebx,%ebx /* swapgs needed? */ jnz nmi_restore - testl $3,CS(%rsp) - jnz nmi_userspace nmi_swapgs: SWAPGS_UNSAFE_STACK nmi_restore: RESTORE_ALL 8 jmp irq_return -nmi_userspace: - GET_THREAD_INFO(%rcx) - movl TI_flags(%rcx),%ebx - andl $_TIF_WORK_MASK,%ebx - jz nmi_swapgs - movq %rsp,%rdi /* &pt_regs */ - call sync_regs - movq %rax,%rsp /* switch stack for scheduling */ - testl $_TIF_NEED_RESCHED,%ebx - jnz nmi_schedule - movl %ebx,%edx /* arg3: thread flags */ - ENABLE_INTERRUPTS(CLBR_NONE) - xorl %esi,%esi /* arg2: oldset */ - movq %rsp,%rdi /* arg1: &pt_regs */ - call do_notify_resume - DISABLE_INTERRUPTS(CLBR_NONE) - jmp nmi_userspace -nmi_schedule: - ENABLE_INTERRUPTS(CLBR_ANY) - call schedule - DISABLE_INTERRUPTS(CLBR_ANY) - jmp nmi_userspace CFI_ENDPROC -#else - jmp paranoid_exit - CFI_ENDPROC -#endif END(nmi) ENTRY(ignore_sysret) -- 1.7.7.3 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/