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

[Xin Li <xin@zytor.com>]

On 2/7/2024 10:30 AM, Paul E. McKenney wrote:
> 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@debian.org>
> 
> If someone else is taking this:
> 
> Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
> 
> (I am queueing it for testing in any case.)

Does this fix need to be backported?

Commit 344da544f177 has a date on Dec 16 2022.

> 
>> ---
>>   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
>>
> 

-- 
Thanks!
     Xin