Re: [PATCH] x86/nmi: Fix "in NMI handler" check

[Paul E. McKenney]

On Wed, Feb 07, 2024 at 08:52:35AM -0800, Breno Leitao wrote:
> Commit 344da544f177 ("x86/nmi: Print reasons why backtrace NMIs are
> ignored") creates a super nice framework to diagnose NMIs.
> 
> Every time nmi_exc() is called, it increments a per_cpu counter
> (nsp->idt_nmi_seq). At its exit, it also increments the same counter.
> Looking at this counter, you can see how many times that function was
> called (dividing by 2), and, if the function is still being executed, by
> checking the idt_nmi_seq's last bit.
> 
> On the check side (nmi_backtrace_stall_check()), that variable is
> queried to check if the NMI is still being executed, but, there is a
> mistake in the bitwise operation. That code wants to check if the last
> bit of the idt_nmi_seq is set or not, but, does the opposite, and check
> for all the other bits, which will always be true after the first
> exc_nmi() executed successfully.
> 
> This appends the misleading string to the dump "(CPU currently in NMI
> handler function)"
> 
> Fix it by checking the last bit, and if it is set, append the string.
> 
> Fixes: 344da544f177 ("x86/nmi: Print reasons why backtrace NMIs are ignored")
> Signed-off-by: Breno Leitao <leitao@xxxxxxxxxx>

If someone else is taking this:

Reviewed-by: Paul E. McKenney <paulmck@xxxxxxxxxx>

(I am queueing it for testing in any case.)

> ---
>  arch/x86/kernel/nmi.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/nmi.c b/arch/x86/kernel/nmi.c
> index 17e955ab69fe..6e738ad474dc 100644
> --- a/arch/x86/kernel/nmi.c
> +++ b/arch/x86/kernel/nmi.c
> @@ -639,7 +639,7 @@ void nmi_backtrace_stall_check(const struct cpumask *btp)
>  			msgp = nmi_check_stall_msg[idx];
>  			if (nsp->idt_ignored_snap != READ_ONCE(nsp->idt_ignored) && (idx & 0x1))
>  				modp = ", but OK because ignore_nmis was set";
> -			if (nmi_seq & ~0x1)
> +			if (nmi_seq & 0x1)
>  				msghp = " (CPU currently in NMI handler function)";
>  			else if (nsp->idt_nmi_seq_snap + 1 == nmi_seq)
>  				msghp = " (CPU exited one NMI handler function)";
> -- 
> 2.39.3
>