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x86/nmi: Fix the inverse "in NMI handler" check

Prarit Bhargava requested to merge prarit/centos-stream-9:RHEL-30106 into main

JIRA: https://issues.redhat.com/browse/RHEL-30106

commit d54e56f31a34fa38fcb5e91df609f9633419a79a
Author: Breno Leitao leitao@debian.org
Date: Wed Feb 7 08:52:35 2024 -0800

x86/nmi: Fix the inverse "in NMI handler" check  

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.  By  
reading this counter it can be seen 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 least significant 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 least significant bit of  
the idt_nmi_seq is set or not, but does the opposite, and checks 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 least significant 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>  
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>  
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>  
Cc: stable@vger.kernel.org  
Link: https://lore.kernel.org/r/20240207165237.1048837-1-leitao@debian.org

Signed-off-by: Prarit Bhargava prarit@redhat.com

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